diff --git a/compression/python/pytree/PYTREE_README.md b/compression/python/pytree/PYTREE_README.md
new file mode 100644
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+++ b/compression/python/pytree/PYTREE_README.md
@@ -0,0 +1,8 @@
+
+# General Remarks about the "PyTree" Abstraction
+
+The pytree wrangling code in this project does not use any of the existing
+"pytree" modules. The deeper reason here is that our approach is based on an
+analysis of the notion that emphasizes deeper underlying principles. This is
+being discussed internally at the time of this writing.
+
diff --git a/compression/python/pytree/build_model_file_for_cpp_binary.py b/compression/python/pytree/build_model_file_for_cpp_binary.py
new file mode 100644
index 0000000..d039639
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+++ b/compression/python/pytree/build_model_file_for_cpp_binary.py
@@ -0,0 +1,275 @@
+"""Ad-hoc glue code for building the griffin model-file for the C++ binary.
+
+Usage:
+
+python3 -m venv $HOME/clients/griffin-venv
+
+. $HOME/clients/griffin-venv/bin/activate
+
+python3 -m pip install -r requirements.txt
+
+time python3 build_model_file_for_cpp_binary.py \
+  $HOME/GRIFFIN/model_data \
+  cpp_load_log.txt /tmp/G2B.data
+
+real    3m5.821s
+user    2m9.205s
+sys     2m46.720s
+
+./compress_weights --weights /tmp/G2B.data --model gr2b-it \
+  --compressed_weights /tmp/G2B.compressed
+./gemma --tokenizer tokenizer.spm --weights /tmp/G2B.compressed \
+  --model gr2b-it
+
+Weights for the recurrent-gemma model that can be converted with this script
+can be found at:
+
+  https://www.kaggle.com/models/google/recurrentgemma/flax/2b-it
+"""
+
+import pprint
+import re
+import sys
+
+from typing import Any, Mapping
+
+import numpy
+
+import orbax.checkpoint
+
+import ml_model_transforms
+import pytree_transforms
+
+
+def _fn_identity(x): return x
+
+
+def _fn_transpose(x): return x.T
+
+
+def _fn_transpose_all_heads(x): return x.transpose(0, 2, 1)
+
+
+def _fn_scaled_softplus(a):
+  return -8 * numpy.logaddexp(a, 0)
+
+
+def _fn_attention_moveaxis(a):
+  return a.reshape(10, 256, 2560).transpose(0, 2, 1)
+
+
+def _aspec(pieces=(), transforms=()):
+  """Short-hand array-save-specification.
+
+  Args:
+    pieces: Sequence of key-sequences identifying an array.
+    transforms: Sequence of transformations, indexed in
+      parallel to `pieces`, to apply to data arrays prior to saving.
+      Will be padded with identity-transformations to the length of `pieces`.
+
+  Returns:
+    Specification as for use in _LAYETR_NAME_MAPPING.
+  """
+  # `zip` trims to shortest sequence, so this amounts to using
+  # default-transforms.
+  # tuple() since we need a Sequence here, not a stateful-iterator zip_object.
+  return tuple(zip(pieces, list(transforms) + [_fn_identity] * len(pieces)))
+
+
+_LAYER_NAME_MAPPING = pytree_transforms.deep_freeze({
+    # Recurrent Layer
+    'griffin_linear_x_w': _aspec(
+        [('recurrent_block', 'linear_x', 'kernel')],
+        [_fn_transpose]),
+    'griffin_linear_x_biases': _aspec(
+        [('recurrent_block', 'linear_x', 'bias')]),
+    'griffin_linear_y_w': _aspec(
+        [('recurrent_block', 'linear_y', 'kernel')],
+        [_fn_transpose]),
+    'griffin_linear_y_biases': _aspec(
+        [('recurrent_block', 'linear_y', 'bias')]),
+    'griffin_linear_out_w': _aspec(
+        [('recurrent_block', 'linear_out', 'kernel')],
+        [_fn_transpose]),
+    'griffin_linear_out_biases': _aspec(
+        [('recurrent_block' ,'linear_out', 'bias')]),
+    'griffin_conv_w': _aspec(
+        [('recurrent_block', 'conv_1d', 'w')]),
+    'griffin_conv_biases': _aspec(
+        [('recurrent_block', 'conv_1d', 'b')]),
+    'griffin_gate_w': _aspec(
+        [('recurrent_block', 'rg_lru', 'input_gate', 'w'),
+         ('recurrent_block', 'rg_lru', 'a_gate', 'w')],
+        [_fn_transpose_all_heads, _fn_transpose_all_heads]),
+    'griffin_gate_biases': _aspec(
+        [('recurrent_block', 'rg_lru', 'input_gate', 'b'),
+         ('recurrent_block', 'rg_lru', 'a_gate', 'b')]),
+    'griffin_a': _aspec(
+        [('recurrent_block', 'rg_lru', 'a_param')],
+        [_fn_scaled_softplus]),
+    # Attention Layer
+    'qkv_einsum_w': _aspec(
+        [('attention_block', 'proj_q', 'kernel'),
+         ('attention_block', 'proj_k', 'kernel'),
+         ('attention_block', 'proj_v', 'kernel'),
+         ],
+        [_fn_transpose, _fn_transpose, _fn_transpose]),
+    'attn_vec_einsum_w': _aspec(
+        [('attention_block', 'proj_final', 'kernel')],
+        [_fn_attention_moveaxis]),
+    'attention_output_biases': _aspec(
+        [('attention_block', 'proj_final', 'bias')]),
+    # Common
+    'pre_attention_norm_scale': _aspec(
+        [('temporal_pre_norm', 'scale')]),
+    'pre_ffw_norm_scale': _aspec(
+        [('channel_pre_norm', 'scale')]),
+    'gating_einsum_w': _aspec(
+        [('mlp_block', 'ffw_up', 'w')],
+        [_fn_transpose_all_heads]),
+    'ffw_gating_biases': _aspec(
+        [('mlp_block', 'ffw_up', 'b')]),
+    'linear_w': _aspec(
+        [('mlp_block', 'ffw_down', 'kernel')],
+        [_fn_transpose]),
+    'ffw_output_biases': _aspec(
+        [('mlp_block', 'ffw_down', 'bias')]),
+    # Other
+    'embedder_input_embedding': _aspec(
+        [('embedder', 'input_embedding')]),
+    'final_norm_scale': _aspec(
+        [('final_norm', 'scale')]),
+})
+
+
+def process_param_line(line : str) -> tuple[None | str, int, str]:
+  """Processes a "loading parameters" log-line from the griffin binary."""
+  # This is slightly more permissive than strictly needed, to also handle
+  # some earlier form of the output.
+  matched = re.match(
+      r'(?a)Loading Parameters:? \('
+      r'(?:layer=(?P<layer>\d+), )?'
+      r'size (?P<size>\d+)\):? '
+      r'(?P<tag>\S+)',
+      line)
+  if not matched:
+    return None
+  layer = matched['layer']
+  wanted_size = int(matched['size'])
+  cpp_tag = matched['tag']
+  return matched['layer'], int(matched['size']), matched['tag']
+
+
+def collect_pytree_keys(param_lines):
+  """Collects all the pytree keys and transforms for model-serialization."""
+  pytree_keys = []
+  array_transforms = []
+  unsatisfied = []
+  for maybe_spec in map(process_param_line, param_lines):
+    if not maybe_spec: continue  # Skip non-parameter lines.
+    layer, wanted_size, cpp_tag = maybe_spec
+    pytree_key_tails_and_transforms = _LAYER_NAME_MAPPING.get(cpp_tag, ())
+    if not pytree_key_tails_and_transforms:
+      unsatisfied.append((layer, cpp_tag))
+    else:
+      for key_tail, array_transform in pytree_key_tails_and_transforms:
+        pytree_keys.append(
+            key_tail if layer is None
+            else (f'blocks.{layer}',) + key_tail)
+        array_transforms.append(array_transform)
+  return pytree_keys, array_transforms, unsatisfied
+
+
+class UnsatisfiedArrayLoadsError(ValueError):
+  """Some array-loads could not be satisfied."""
+
+
+def flatten_model_for_cpp_binary(tree,
+                                 cpp_expectations_logfile_path : str,
+                                 out_path : str,
+                                 unsatisfied_ok : bool = False
+                                 ):
+  """Produces a model-parameters file readable by the C++ binary.
+
+  Args:
+    tree: The pytree with model-parameters.
+    cpp_expectations_logfile_path:
+      Path to a logfile produced by the C++ binary that shows
+      the expected array-order.
+    out_path: Path to the model-weights file to be written.
+    unsatisfied_ok: If true, we ignore the presence of unsatisfied
+      array-loads and write a model-parameters file that skips these pieces.
+      This will lead to an unusable model-parameters file which however
+      still might be useful for other analysis.
+
+  Returns:
+    Tuple `(unknown_keys, missing_keys)`, where `unknown_keys`
+    is a sequence of `(layer_or_None, name)` descriptions of the keys
+    in the C++ log that could not be satisfied, and `missing_keys`
+    is a sequence of linearized pytree key-sequences for keys
+    not found in the checkpoint.
+
+  Raises:
+    UnsatisfiedArrayLoadsError: If some of the expected arrays
+      could not be included in the output and `unsatisfied_ok`
+      is false.
+  """
+  with open(cpp_expectations_logfile_path, 'rt') as h_log:
+    pytree_keys, array_transforms, unknown_keys = collect_pytree_keys(
+        list(h_log))
+  rank_by_pytree_key = {k: n for n, k in enumerate(pytree_keys)}
+  array_transform_by_pytree_key = dict(zip(pytree_keys, array_transforms))
+  #
+  model_contents = ml_model_transforms.model_contents(tree)
+  missing_keys = set(pytree_keys) - model_contents.keys()
+  if (unknown_keys or missing_keys) and not unsatisfied_ok:
+    raise ValueError(
+      f'Unsatisfied loads: unknown_keys: {unknown_keys!r}, '
+      f'missing keys: {sorted(missing_keys)!r}')
+  ml_model_transforms.model_save(
+    tree,
+    filepath_stem=out_path,
+    data_suffix='',
+    manifest_suffix=None,
+    array_transform_by_pytree_key=array_transform_by_pytree_key,
+    key=rank_by_pytree_key.get,
+    report=lambda line: print(line, file=sys.stderr),
+    byte_align=1)
+  return tuple(unknown_keys), tuple(sorted(missing_keys))
+
+
+def main(args):
+  """Creates the model-file.
+
+  Args:
+    sys.argv[] parameters from command line sans the leading one.
+
+  Returns:
+    The pytree with all the de-serialized variables, such as for convenient
+    `python3 -i` inspection.
+  """
+  try:
+    model_dir, cpp_load_log, out_path = args
+  except Exception:
+    sys.exit(f'Usage: {__file__} [model_dir] [cpp_load_log] [output_filename]')
+  pattern = ("recurrent", "recurrent", "attention")
+  orbax_checkpointer = orbax.checkpoint.PyTreeCheckpointer()
+  variables = orbax_checkpointer.restore(model_dir)
+  if sorted(variables) == ['params']:
+    print('Warning: Using `variables["params"]` as tree-root.', file=sys.stderr)
+    variables_to_use = variables['params']
+  else:
+    variables_to_use = variables
+  unknown, missing = flatten_model_for_cpp_binary(variables_to_use,
+                                                  cpp_load_log,
+                                                  out_path,
+                                                  unsatisfied_ok=True)
+  print('Model file saved.\n'
+        f'# unknown:\n{pprint.pformat(unknown)}\n'
+        f'# missing:\n{pprint.pformat(missing)}')
+  return variables
+
+
+if __name__ == '__main__':
+  # Return value assignment is for `python3 -i ...` inspection.
+  pytree = main(sys.argv[1:])
diff --git a/compression/python/pytree/cpp_load_log.txt b/compression/python/pytree/cpp_load_log.txt
new file mode 100644
index 0000000..cc33394
--- /dev/null
+++ b/compression/python/pytree/cpp_load_log.txt
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+Loading Parameters (size 2622750720): embedder_input_embedding
+Loading Parameters (size 10240): final_norm_scale
+Loading Parameters: (layer=0, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=0, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=0, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=0, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=0, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=0, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=0, size 40960) griffin_conv_w
+Loading Parameters: (layer=0, size 10240) griffin_conv_biases
+Loading Parameters: (layer=0, size 5242880) griffin_gate_w
+Loading Parameters: (layer=0, size 20480) griffin_gate_biases
+Loading Parameters: (layer=0, size 10240) griffin_a
+Loading Parameters: (layer=0, size 157286400) gating_einsum_w
+Loading Parameters: (layer=0, size 78643200) linear_w
+Loading Parameters: (layer=0, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=0, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=0, size 61440) ffw_gating_biases
+Loading Parameters: (layer=0, size 10240) ffw_output_biases
+Loading Parameters: (layer=1, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=1, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=1, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=1, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=1, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=1, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=1, size 40960) griffin_conv_w
+Loading Parameters: (layer=1, size 10240) griffin_conv_biases
+Loading Parameters: (layer=1, size 5242880) griffin_gate_w
+Loading Parameters: (layer=1, size 20480) griffin_gate_biases
+Loading Parameters: (layer=1, size 10240) griffin_a
+Loading Parameters: (layer=1, size 157286400) gating_einsum_w
+Loading Parameters: (layer=1, size 78643200) linear_w
+Loading Parameters: (layer=1, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=1, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=1, size 61440) ffw_gating_biases
+Loading Parameters: (layer=1, size 10240) ffw_output_biases
+Loading Parameters: (layer=2, size 26214400) attn_vec_einsum_w
+Loading Parameters: (layer=2, size 78643200) qkv_einsum_w
+Loading Parameters: (layer=2, size 157286400) gating_einsum_w
+Loading Parameters: (layer=2, size 78643200) linear_w
+Loading Parameters: (layer=2, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=2, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=2, size 61440) ffw_gating_biases
+Loading Parameters: (layer=2, size 10240) ffw_output_biases
+Loading Parameters: (layer=2, size 10240) attention_output_biases
+Loading Parameters: (layer=3, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=3, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=3, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=3, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=3, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=3, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=3, size 40960) griffin_conv_w
+Loading Parameters: (layer=3, size 10240) griffin_conv_biases
+Loading Parameters: (layer=3, size 5242880) griffin_gate_w
+Loading Parameters: (layer=3, size 20480) griffin_gate_biases
+Loading Parameters: (layer=3, size 10240) griffin_a
+Loading Parameters: (layer=3, size 157286400) gating_einsum_w
+Loading Parameters: (layer=3, size 78643200) linear_w
+Loading Parameters: (layer=3, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=3, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=3, size 61440) ffw_gating_biases
+Loading Parameters: (layer=3, size 10240) ffw_output_biases
+Loading Parameters: (layer=4, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=4, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=4, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=4, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=4, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=4, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=4, size 40960) griffin_conv_w
+Loading Parameters: (layer=4, size 10240) griffin_conv_biases
+Loading Parameters: (layer=4, size 5242880) griffin_gate_w
+Loading Parameters: (layer=4, size 20480) griffin_gate_biases
+Loading Parameters: (layer=4, size 10240) griffin_a
+Loading Parameters: (layer=4, size 157286400) gating_einsum_w
+Loading Parameters: (layer=4, size 78643200) linear_w
+Loading Parameters: (layer=4, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=4, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=4, size 61440) ffw_gating_biases
+Loading Parameters: (layer=4, size 10240) ffw_output_biases
+Loading Parameters: (layer=5, size 26214400) attn_vec_einsum_w
+Loading Parameters: (layer=5, size 78643200) qkv_einsum_w
+Loading Parameters: (layer=5, size 157286400) gating_einsum_w
+Loading Parameters: (layer=5, size 78643200) linear_w
+Loading Parameters: (layer=5, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=5, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=5, size 61440) ffw_gating_biases
+Loading Parameters: (layer=5, size 10240) ffw_output_biases
+Loading Parameters: (layer=5, size 10240) attention_output_biases
+Loading Parameters: (layer=6, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=6, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=6, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=6, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=6, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=6, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=6, size 40960) griffin_conv_w
+Loading Parameters: (layer=6, size 10240) griffin_conv_biases
+Loading Parameters: (layer=6, size 5242880) griffin_gate_w
+Loading Parameters: (layer=6, size 20480) griffin_gate_biases
+Loading Parameters: (layer=6, size 10240) griffin_a
+Loading Parameters: (layer=6, size 157286400) gating_einsum_w
+Loading Parameters: (layer=6, size 78643200) linear_w
+Loading Parameters: (layer=6, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=6, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=6, size 61440) ffw_gating_biases
+Loading Parameters: (layer=6, size 10240) ffw_output_biases
+Loading Parameters: (layer=7, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=7, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=7, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=7, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=7, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=7, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=7, size 40960) griffin_conv_w
+Loading Parameters: (layer=7, size 10240) griffin_conv_biases
+Loading Parameters: (layer=7, size 5242880) griffin_gate_w
+Loading Parameters: (layer=7, size 20480) griffin_gate_biases
+Loading Parameters: (layer=7, size 10240) griffin_a
+Loading Parameters: (layer=7, size 157286400) gating_einsum_w
+Loading Parameters: (layer=7, size 78643200) linear_w
+Loading Parameters: (layer=7, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=7, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=7, size 61440) ffw_gating_biases
+Loading Parameters: (layer=7, size 10240) ffw_output_biases
+Loading Parameters: (layer=8, size 26214400) attn_vec_einsum_w
+Loading Parameters: (layer=8, size 78643200) qkv_einsum_w
+Loading Parameters: (layer=8, size 157286400) gating_einsum_w
+Loading Parameters: (layer=8, size 78643200) linear_w
+Loading Parameters: (layer=8, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=8, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=8, size 61440) ffw_gating_biases
+Loading Parameters: (layer=8, size 10240) ffw_output_biases
+Loading Parameters: (layer=8, size 10240) attention_output_biases
+Loading Parameters: (layer=9, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=9, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=9, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=9, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=9, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=9, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=9, size 40960) griffin_conv_w
+Loading Parameters: (layer=9, size 10240) griffin_conv_biases
+Loading Parameters: (layer=9, size 5242880) griffin_gate_w
+Loading Parameters: (layer=9, size 20480) griffin_gate_biases
+Loading Parameters: (layer=9, size 10240) griffin_a
+Loading Parameters: (layer=9, size 157286400) gating_einsum_w
+Loading Parameters: (layer=9, size 78643200) linear_w
+Loading Parameters: (layer=9, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=9, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=9, size 61440) ffw_gating_biases
+Loading Parameters: (layer=9, size 10240) ffw_output_biases
+Loading Parameters: (layer=10, size 26214400) griffin_linear_x_w
+Loading Parameters: (layer=10, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=10, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=10, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=10, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=10, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=10, size 40960) griffin_conv_w
+Loading Parameters: (layer=10, size 10240) griffin_conv_biases
+Loading Parameters: (layer=10, size 5242880) griffin_gate_w
+Loading Parameters: (layer=10, size 20480) griffin_gate_biases
+Loading Parameters: (layer=10, size 10240) griffin_a
+Loading Parameters: (layer=10, size 157286400) gating_einsum_w
+Loading Parameters: (layer=10, size 78643200) linear_w
+Loading Parameters: (layer=10, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=10, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=10, size 61440) ffw_gating_biases
+Loading Parameters: (layer=10, size 10240) ffw_output_biases
+Loading Parameters: (layer=11, size 26214400) attn_vec_einsum_w
+Loading Parameters: (layer=11, size 78643200) qkv_einsum_w
+Loading Parameters: (layer=11, size 157286400) gating_einsum_w
+Loading Parameters: (layer=11, size 78643200) linear_w
+Loading Parameters: (layer=11, size 10240) pre_attention_norm_scale
+Loading Parameters: (layer=11, size 10240) pre_ffw_norm_scale
+Loading Parameters: (layer=11, size 61440) ffw_gating_biases
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+Loading Parameters: (layer=11, size 10240) attention_output_biases
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+Loading Parameters: (layer=12, size 10240) griffin_linear_x_biases
+Loading Parameters: (layer=12, size 26214400) griffin_linear_y_w
+Loading Parameters: (layer=12, size 10240) griffin_linear_y_biases
+Loading Parameters: (layer=12, size 26214400) griffin_linear_out_w
+Loading Parameters: (layer=12, size 10240) griffin_linear_out_biases
+Loading Parameters: (layer=12, size 40960) griffin_conv_w
+Loading Parameters: (layer=12, size 10240) griffin_conv_biases
+Loading Parameters: (layer=12, size 5242880) griffin_gate_w
+Loading Parameters: (layer=12, size 20480) griffin_gate_biases
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+Loading Parameters: (layer=19, size 61440) ffw_gating_biases
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+Loading Parameters: (layer=25, size 5242880) griffin_gate_w
+Loading Parameters: (layer=25, size 20480) griffin_gate_biases
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diff --git a/compression/python/pytree/ml_model_transforms.py b/compression/python/pytree/ml_model_transforms.py
new file mode 100644
index 0000000..3605c07
--- /dev/null
+++ b/compression/python/pytree/ml_model_transforms.py
@@ -0,0 +1,371 @@
+"""Transformations for python-trees representing the parameters of a ML model.
+
+Important: This module assumes that byte-order is the same on the
+machine that serializes data and the machine that deserializes
+data. If, for example, numpy-data gets dumped, respectively loaded,
+with a dtype-specification of numpy.float32, on-file byte-order
+will be host byte order.
+
+"""
+
+import ast
+import hashlib
+import itertools
+import pprint
+import sys
+import time
+from typing import Any, Callable, Iterable, Iterator, Mapping, TypeVar
+
+import numpy
+import pytree_transforms
+
+
+NT = TypeVar('NT')
+
+
+def ml_model_leaf_summary(path, x, sep=', '):
+  """Produces a textual summary of a leaf-node and its path.
+
+  Args:
+    path: The path-to-root, as a reverse-order recursive
+      pair of path-components, with `()` as root.
+    x: The leaf-object.
+    sep: the separator between description-elements.
+      Default ', ' allows for convenient line-by-line processing
+      (such as via grep, perl -ne, etc.), but using e.g. sep=',\n  '
+      might be more useful for human consumption.
+
+  Returns:
+    A human-readable string providing information about the node.
+  """
+  # Using `repr` for path-components to get a faithful presentation.
+  # (...which still however would be somewat painful to correctly
+  # split into components.)
+  path_str = ','.join(map(repr,
+                          pytree_transforms.linearize_revtuple_path(path)))
+  tx = type(x)
+  mod = tx.__module__  # Either a module or a string like 'builtins'.
+  modname = mod if isinstance(mod, str) else mod.__name__
+  type_str = f'{modname}.{tx.__qualname__}'
+  try:
+    # `numpy.ndarray` instances have a `.data` property that gives access
+    # to a buffer via which we can hashlib-fingerprint the numerical
+    # contents. We here simply try to produce a fingerprint and also look
+    # up the .dtype of the object. Technically, there is a somewhat-unsound
+    # assumption here that if these operations succeed, we are indeed looking
+    # at a ndarray or sufficiently similar object for these operations to
+    # make sense. As the output is declared "for human consumption", this
+    # fishiness is not a problem.
+    fp = hashlib.sha256(x.data).hexdigest()
+    start = list(itertools.islice(x.flat, 5))
+    stats_str = (
+        f'min={numpy.min(x):.6g}, max={numpy.max(x):.6g}, '
+        f'mean={numpy.mean(x):.6g}, std={numpy.std(x):.6g}')
+    return (f'{path_str:60s}: <{type_str}{sep}'
+            f'fp=0x{fp}{sep}{stats_str}{sep}shape={x.shape}, '
+            f'dtype={x.dtype}{sep}start={start}>')
+  except (AttributeError, ValueError, TypeError):
+    # Fallback - trying to include information about the data-content
+    # of a likely-numerical-array failed.
+    return f'{path_str:60s}: {type_str}({repr(x)})'
+
+
+# A specialized node-handler.
+# Interface follows node-handler expectations defined in pytree_transforms.
+def _ml_model_tree_node_handler(path: tuple, node : NT) -> (
+    None | tuple[Callable[[Iterable[tuple[Any, NT]]], NT],
+                 Iterator[tuple[Any, NT]]]):
+  """Processes a tree-node as required by pytree-iteration and -mapping.
+
+  Args:
+    path: revtuple path to the current node.
+    node: a tree-node in a ML-model tree that is recursively
+      built out of `numpy.ndarray` leaf-values and dicts mapping
+      node-name string-keys to other such nodes representing subtrees -
+      and nothing else.
+
+  Returns:
+    `None` if the tree-node is to be regarded as a leaf, otherwise
+    a pair `(rebuilder, iterator)`, where `iterator` iterates
+    over the data-content of the node, each item represented as a pair
+    of `(lookup_path_item, value_item)`, and `rebuilder` is a function
+    which, when applied to `iterator` or any iterable with the same
+    elements, returns a node that is equivalent to the original.
+
+  Raises:
+    NotAMLModelTreeNodeError: If the tree contains a node that is neither
+      a `dict` nor a `numpy.ndarray` instance.
+  """
+  # The astute reader will notice that we are doing something fishy
+  # here - this code could not be translated to Haskell as-is, since
+  # `NT` cannot actually be a proper type-variable in the sense
+  # of parametric polymorphism.
+  del path  # Unused.
+  if isinstance(node, dict):
+    return dict, iter(node.items())
+  if isinstance(node, numpy.ndarray):
+    return None
+  raise pytree_transforms.NotAMLModelTreeNodeError(
+      f'Type of bad node: {type(node)}')
+
+
+def _ml_model_extract_leaf_transform(
+        path: pytree_transforms.RevTuplePath,
+        leaf: Any):
+  """Maps an array-leaf to a pair `(full_path, lambda: array)`.
+
+  The computation that produces the leaf-value is lazified underneath
+  a `lambda`, since if we e.g. performed a memory-expensive
+  transformation (such as some dtype-changes) directly at this point,
+  then going from an iterator over tree-items for one-by-one
+  consumption to a list of these items would have all the
+  dtype-transformed values around simultaneously. We want to avoid
+  situations where we can do nothing about having multiple variants
+  of the data simultaneously in memory.
+  """
+  # Hack: If we are encountering a `bfloat16` numpy-array,
+  # we pretend to have the data as a numpy.float32 array,
+  # since that's about all that contemporary CPUs can process
+  # efficiently here.
+  linearized_path = pytree_transforms.linearize_revtuple_path(path)
+  try:
+    # We have to use some trickery to detect `bfloat16`.
+    if leaf.dtype.descr[-1] == ('', '<V2'):
+      return linearized_path, lambda: leaf.astype(numpy.float32)
+    else:
+      return linearized_path, lambda: leaf
+  except Exception:
+    return linearized_path, lambda: leaf
+
+
+# Here, we cannot properly specify the return-type, since this can
+# either be a leaf-type or something recursively-defined.
+def revtuple_autovifify_from_linear(
+        keys_and_vals: Iterable[tuple[Any, Any]]) -> Any:
+  """Performs perl-style autovivification on a nested-dict tree.
+
+  Args:
+    keys_and_vals: An iterable of pairs `(key_path, value)`, where
+      `key_path` is a sequence of keys to be used to navigate to
+      the result via iterative dict-lookup, left-to-right.
+      Must not have duplicate keys, and must not more than one key if
+      an empty-sequence key is present. If this iterable is an
+      iterator, it will be fully exhausted on successful execution.
+
+  Returns:
+    An object representing a nested-dict structure such that
+    for every `key_path` from `keys_and_vals`, recursive-dict-lookup
+    on the elements of that path starting from this object will
+    produce the corresponding value. An empty `keys_and_vals`
+    set will return `{}`. Every dict in the nested return-value
+    that has been populated by autovivification is newly allocated.
+  """
+  # Code structure is a bit gnarly here due to f(keys_and_vals=[((), x)])
+  # having to evaluate to x and not a dict.
+  # There may be ways to prettify/simplify this.
+  result = None
+  empty = {}
+  for linear_path, val in keys_and_vals:
+    if linear_path == ():
+      if result is not None:
+        raise ValueError('Root-value seen alongside other values.')
+      result = val
+    else:
+      if result is None:
+        result = {}
+      elif type(result) is not dict:
+        # We already did encounter a root-value.
+        raise ValueError('Root-value seen alongside other values.')
+      cursor = result
+      for n in range(len(linear_path) - 1):
+        cursor = cursor.setdefault(linear_path[n], empty)
+        if cursor is empty:
+          # Regenerate `empty` if we just used it up.
+          empty = {}
+      cursor[linear_path[-1]] = val
+  return {} if result is None else result
+
+
+def model_overview(tree, out=None) -> None:
+  """Prints a human-readable overview to `(out or sys.stdout)`."""
+  actual_out = out or sys.stdout
+  for line in pytree_transforms.pytree_leaf_iter(
+      tree, ml_model_leaf_summary,
+      _ml_model_tree_node_handler):
+    print(line, file=actual_out)
+
+
+def model_contents(tree) -> Mapping[tuple[str, ...], Any]:
+  """Maps a model to a {pytree_keys: data_array} mapping.
+
+  Args:
+    tree: The ML-model parameter-tree, built recursively out of
+      dict-instances with numpy.ndarray instances as leaves.
+
+  Returns:
+    A mapping from linearized pytree-key-sequence tuple to the corresponding
+    leaf-value.
+  """
+  def leaf_transform(revtuple_path, leaf):
+    return pytree_transforms.linearize_revtuple_path(revtuple_path), leaf
+  return dict(
+      pytree_transforms.pytree_leaf_iter(
+          tree, leaf_transform, _ml_model_tree_node_handler))
+
+
+def _fn_identity(x): return x
+
+
+def model_save(tree,
+               filepath_stem: str,
+               data_suffix: str = '.data',
+               manifest_suffix: str | None = '.manifest',
+               key: Callable[[tuple[str, ...]], Any] | None = None,
+               array_transform_by_pytree_key: (
+                   Mapping[tuple[str, ...],
+                           Callable[[numpy.ndarray], numpy.ndarray]] |
+                   None) = None,
+               report: Callable[[str], None] | None = None,
+               byte_align: int = 8) -> tuple[int, float]:
+  """Saves the content of a ML-model parameter-tree to filesystem.
+
+  After successful execution, the file f"{filepath_stem}.data"
+  will hold the combined numerical model-parameters, and
+  f"{filepath_stem}.manifest" will contain the key for interpreting
+  (and rebuilding) the data.
+
+  Args:
+    tree: The ML-model parameter-tree, built recursively out of
+      dict-instances with numpy.ndarray instances as leaves.
+    filepath_stem: Filesystem location for data.
+    data_suffix: Suffix to use for the data file.
+    manifest_suffix: Either `None`, in which case no manifest-file
+      will get written, or the suffix for the manifest-file.
+    key: `None` or a key-function that will be applied to the linear model-path
+      and used for sorting the data arrays by increasing value of the
+      key-function. If the key-function returns `None` on an item,
+      then this item is not included.
+    array_transform_by_pytree_key: Optional mapping from pytree-key
+      to an array-to-array transformation function to apply to the array
+      prior to serialization.
+    report: Optional callable for logging progress-reports.
+    byte_align: byte-alignment to use for numerical array data.
+      Numerical arrays whose size in bytes is not a multiple of this
+      will get padded to the next full multiple.
+
+  Returns:
+    A pair of `(size, time_sec)`, where `size` is the total byte-size
+    of the `.data` file and `time_sec` is the elapsed time
+    for saving the model, in seconds.
+  """
+  time0 = time.monotonic()
+  if array_transform_by_pytree_key is None:
+    array_transform_by_pytree_key = {}
+  model_lazy_items = (
+      pytree_transforms.pytree_leaf_iter(
+          tree, _ml_model_extract_leaf_transform,
+          _ml_model_tree_node_handler))
+  if key is not None:
+    to_write = [
+        nkv[1:] for nkv in sorted(
+            (nkv for nkv in ((key(path), path, v)
+                             for path, v in model_lazy_items)
+             if nkv[0] is not None), key=lambda nkv: nkv[0])]
+  else:
+    to_write = list(model_lazy_items)
+  #
+  def lazy_arr_path_shape_dtype_size(path_and_lazy_arr):
+    path, lazy_arr = path_and_lazy_arr
+    arr = array_transform_by_pytree_key.get(path, _fn_identity)(lazy_arr())
+    return path, arr.shape, arr.dtype, arr.data.nbytes
+  arrs_path_shape_dtype_nbytes = list(
+      map(lazy_arr_path_shape_dtype_size, to_write))
+  # We need to know the total size of all the data.
+  bytesizes = [nbytes for *_, nbytes in arrs_path_shape_dtype_nbytes]
+  padded_bytesizes = [-(-bytesize // byte_align * byte_align)
+                      for bytesize in bytesizes]
+  offsets = numpy.cumsum([0] + padded_bytesizes)
+  membuf = numpy.memmap(filepath_stem + data_suffix,
+                        mode='w+', shape=offsets[-1])
+  try:
+    for (path, shape, dtype, nbytes), offset, (_, lazy_arr) in zip(
+        arrs_path_shape_dtype_nbytes, offsets, to_write):
+      # Note that if getting the array from the lazy lambda involved some
+      # computation, such as a copying dtype-change, that computation would
+      # end up being done multiple times here - including once above, to compute
+      # byte-sizes, and once more here.
+      transformed_arr = array_transform_by_pytree_key.get(
+          path,
+          _fn_identity)(lazy_arr())
+      membuf[offset : offset + nbytes] = numpy.frombuffer(
+          transformed_arr.ravel().data, 'u1')
+      if report is not None:
+        samples = ', '.join(map(str, transformed_arr.ravel()[:5]))
+        report(f'# Adding: {path!r}\n  bytes: {nbytes:10d}, '
+               f'shape: {shape!r:30},\n  start: [{samples}, ...]')
+      transformed_arr = None  # Drop memory references to numerical arrays ASAP.
+  finally:
+    if membuf is not None:
+      membuf.flush()
+      # NumPy wart: the memory-buffer is a resource that conceptually
+      # should be .close()able - since mmap()ing holds on to a
+      # file descriptor. However, it looks as if that clean-up were done
+      # in the "finalizer", despite that having meanwhile been widely
+      # understood as dubious practice. So, the best we can do here is
+      # to explicitly and clearly remove our reference to the instance.
+      del membuf
+  if manifest_suffix is not None:
+    # We still have to serialize the data that allows us to reconstruct
+    # a tree that is equivalent to the original.
+    manifest_data = [
+        dict(path=path,
+             dtype=dtype.descr[-1][-1],
+             shape=shape,
+             nbytes=nbytes,
+             offset=offset)
+        for (path, shape, dtype, nbytes), offset in zip(
+            arrs_path_shape_dtype_nbytes, offsets)]
+    with open(filepath_stem + '.manifest', 'wt') as h_manifest:
+      pprint.pprint(manifest_data, stream=h_manifest)
+  time_taken = time.monotonic() - time0
+  return offsets[-1], time_taken
+
+
+def model_load(filepath_stem, mmapped=True):
+  """Loads a model saved by `model_save`.
+
+  Tries to load the model from f"{filepath_stem}.data"
+  and f"{filepath_stem}.manifest".
+
+  Args:
+    filepath_stem: The model location on the filesystem.
+    mmapped: Whether data-arrays will be slices of a
+      `numpy.memmap` mapped buffer, to be paged in
+      on demand only, or in-memory copies of the data.
+  Returns:
+    A dict/numpy.ndarray tree representation of the model,
+    equivalent to the original model.
+  """
+  with open(filepath_stem + '.manifest', 'rt') as h_manifest:
+    manifest = ast.literal_eval(h_manifest.read())
+  membuf = numpy.memmap(filepath_stem + '.data', mode='r+')
+  paths_and_arrays = []
+  for item in manifest:
+    path = item['path']
+    dtype = numpy.dtype(item['dtype'])
+    shape = item['shape']
+    nbytes = item['nbytes']
+    offset = item['offset']
+    data_array = numpy.frombuffer(membuf[offset : offset + nbytes].data,
+                                  dtype=dtype).reshape(shape)
+    paths_and_arrays.append(
+        (path,
+         data_array if mmapped else data_array.copy()))
+  # At this point, the memory-buffer is no longer needed. Still, if
+  # data-arrays retain references to the underlying data
+  # (i.e. when mmapped=False), this should keep the mapping
+  # - and hence file descriptor - open. We then are in a somewhat
+  # undesirable situation of clean-up of a resource that happens in a
+  # hard-to-predict way releasing a file descriptor.
+  del membuf
+  return revtuple_autovifify_from_linear(paths_and_arrays)
diff --git a/compression/python/pytree/ml_model_transforms_test.py b/compression/python/pytree/ml_model_transforms_test.py
new file mode 100644
index 0000000..9495c87
--- /dev/null
+++ b/compression/python/pytree/ml_model_transforms_test.py
@@ -0,0 +1,92 @@
+"""Basic tests for 'algebraic data type based pytree' transformations."""
+
+
+import io
+import os
+import tempfile
+import unittest
+
+import numpy
+
+import ml_model_transforms
+
+
+def _get_model(prefix):
+  return {
+    prefix + 'a1': numpy.arange(1000, 1024).reshape(6, 4).astype(numpy.float32),
+    prefix + 'a2': numpy.arange(2000, 2048).reshape(6, 8).astype(numpy.float32),
+    prefix + 'b1': {
+      prefix + 'c1': numpy.arange(100, 127).reshape(3, 3, 3).astype(numpy.int8),
+      prefix + 'c2': numpy.arange(100, 128).reshape(7, 4).astype(numpy.float64)
+    }}
+
+
+class MLModeltransformsTest(unittest.TestCase):
+  """Basic correctness validation tests for ML-model transformations."""
+
+  def test_ml_model_leaf_summary(self):
+    """Tests guarantees given by `ml_model_leaf_summary`."""
+    summary = ml_model_transforms.ml_model_leaf_summary(
+      ('a', ()),
+      numpy.arange(1000, 1024).reshape(6, 4).astype(numpy.int16),
+      sep='##')
+    self.assertIn('##', summary)  # Separator is respected.
+    self.assertIn('(6, 4)', summary)  # Shape is mentioned somewhere.
+    self.assertIn('int16', summary)  # dtype is mentioned somewhere.
+
+  def test_revtuple_autovivify_from_linear(self):
+    """Tests guarantees given by `revtuple_autovifify_from_linear`."""
+    with self.subTest(guarantee='empty'):
+      self.assertEqual(
+        ml_model_transforms.revtuple_autovifify_from_linear([]),
+        {})
+    with self.subTest(guarantee='generic'):
+      keys_vals = [(('a', 'b1', 'c1'), 1001),
+                   (('a', 'b2'), 1002),
+                   (('a2',), 1003),
+                   ]
+      self.assertEqual(
+        ml_model_transforms.revtuple_autovifify_from_linear(keys_vals),
+        {'a': {'b1': {'c1': 1001}, 'b2': 1002}, 'a2': 1003})
+
+  def test_model_overview(self):
+    """Tests guarantees given by `model_overview`."""
+    model = _get_model('xyz')
+    out_io = io.StringIO()
+    ml_model_transforms.model_overview(model, out=out_io)
+    overview = out_io.getvalue()
+    self.assertIn('xyz', overview)
+
+  def test_model_contents(self):
+    """Tests guarantees given by `model_contents`."""
+    model = _get_model('pq_')
+    contents = ml_model_transforms.model_contents(model)
+    fingerprints = {k: (a.shape, a.ravel()[:3].tolist())
+                    for k, a in contents.items()}
+    self.assertEqual(fingerprints,
+                     {('pq_a1',): ((6, 4), [1000.0, 1001.0, 1002.0]),
+                      ('pq_a2',): ((6, 8), [2000.0, 2001.0, 2002.0]),
+                      ('pq_b1', 'pq_c1'): ((3, 3, 3), [100, 101, 102]),
+                      ('pq_b1', 'pq_c2'): ((7, 4), [100.0, 101.0, 102.0])})
+
+  def test_model_save_load_basic(self):
+    """Tests basic guarantees given by `model_save` and `model_load`."""
+    # What we care about here is that the round trip works - so
+    # it makes more sense to test saving and loading as one unit.
+    model_orig = _get_model('model_')
+    with tempfile.TemporaryDirectory() as tempdir:
+      filepath_stem = os.path.join(tempdir, 'the_model')
+      total_size, total_time = ml_model_transforms.model_save(model_orig,
+                                                              filepath_stem)
+      self.assertGreater(total_size, 0)
+      self.assertGreater(total_time, 0)
+      model_reloaded = ml_model_transforms.model_load(filepath_stem)
+      contents_orig = ml_model_transforms.model_contents(model_orig)
+      contents_reloaded = ml_model_transforms.model_contents(model_reloaded)
+      self.assertEqual(
+        {k: v.tolist() for k, v in contents_orig.items()},
+        {k: v.tolist() for k, v in contents_reloaded.items()})
+
+
+if __name__ == '__main__':
+  unittest.main()
diff --git a/compression/python/pytree/pytree_transforms.py b/compression/python/pytree/pytree_transforms.py
new file mode 100644
index 0000000..7e065af
--- /dev/null
+++ b/compression/python/pytree/pytree_transforms.py
@@ -0,0 +1,508 @@
+"""Tools for transforming "nested python object" tree data structures.
+
+# Context
+
+The motivation for this module came from ML applications that ought to
+be based on a principled handling of nested Python data structures.
+Having such principled pytree-transforming code available solves
+some other problems, such as doing away with a need to abuse
+tree-mapping for-side-effect-only and having to use a hope-and-pray
+approach to processing very deeply nested values which with a recursive
+approach might trigger a RecursionError.
+
+We specifically want to cover the use case of having ML model
+parameters that are available in a nested Python data structure for
+which there "almost" is a unique-up-to-unique-isomorphism mapping from
+and to this Algebraic Data Type:
+
+`data ModelParams a = Array a | Node [(String, ModelParams a)]`
+
+In this correspondence, `a` is some array-type (perhaps
+`numpy.ndarray`, `jax.numpy.ndarray`, `tf.tensor`, etc.), but the
+data-processing code is effectively entirely agnostic to this, and a
+`Node` is "almost" an associative-list of (key, value) pairs
+representing a Python dict. (Note: The "almost" here is mostly about
+the conceptual wart that assoc-lists can in principle have key
+duplicates, but Python dicts can not. This is however not a problem
+since all we need is the transformation in one direction,
+i.e. whatever data-processing `f` we want to express on the
+model-parameters-pytree, we can express by specifying a "faithful"
+mapping `m` into the above algebraic data type through which every
+such pytree data transform factorizes, i.e. for every `f` we can find
+a `g` such that `f(p) = g(m(p))`.)
+
+## Components
+
+The main workhorse in this module is the `pytree_iter` function that
+maps a "PyTree (such as representing `ModelParams`)" to an iterator
+over values obtained by applying a mapping-function to the "key-path"
+and leaf-value for every leaf, where the "key-path" contains a
+linked-list representation of the reversed sequence of keys from the
+tree-root, with list-nodes being represented by pairs
+`(latest_dict_key, rest_path)`, and the empty path being represented
+by `()`.
+
+For the sake of genericity, `pytree_iter` is built in such a way that
+it actually can handle any kind of traversal of PyTree-trees that do
+represent algebraic data types (note however that some some do not) -
+but for this to make sense, the user must have a way to define how to
+interpret tree-nodes, in particular identify leaves. This requires
+providing a function `node_handler` with the same signature and
+behavior as described below for "node handlers".
+
+Additionally, this module provides mapping-over-pytrees via
+`pytree_map`, which is also built in such a way that it makes the
+correspondence between an algebraic data type and its Python
+nested-tree representation explicit. Despite being powerful and
+flexible, this, however, may in general require a bit more effort to
+wire up, since node-rebuilding can be fairly nontrivial.
+
+Furthermore, as a prominent application, this module provides a simple
+deep-freezing function that translates a nested Python data structure
+to deeply-immutable form.
+
+## Concepts and Conventions
+
+"revtuple representation":
+
+  As we iterate over a tree, we will have to keep track of the
+  path-to-tree-root. Naturally, two sibling nodes `n1` and `n2`
+  will share the same parent-path (being siblings), so it makes
+  sense to use a linked-list-with-shared-tail representation.
+  Python does not have a natural notion for that, so we use
+  recursively-constructed tuples `(node_tag, parent_path)`
+  that represent the path-from-root in-reverse-order, i.e.
+  for a non-empty path `p`, `p[0]` is the node-tag at the
+  deepest nesting level. We call this a "revtuple representation"
+  of the path.
+
+"node handler":
+
+  A node-handler classifies a tree-node as "leaf or other node", and
+  for non-leaf nodes provides information about both its children and
+  how to rebuild it. The behavior of a node-handler function must be
+  in alignment with this docstring:
+
+  '''Processes a tree-node as required by pytree-iteration and -mapping.
+
+  Args:
+    revtuple_path: Revtuple-representation of the path-from-root
+      to the current node.
+    node: a tree-node in a ML-model tree that is recursively
+      built out of leaf-values and other nodes.
+
+  Returns:
+    `None` if the tree-node is to be regarded as a leaf, otherwise
+    a pair `(rebuilder, iterator)`, where `iterator` iterates
+    over the data-content of the node, each item represented as a pair
+    of `(lookup_path_item, value_item)`, and `rebuilder` is a function
+    which, when applied to an iterable of the aforementioned value-items
+    (or some transformation thereof) returns a node that is equivalent
+    to the original (or up to a transformation of the contents).
+
+  Raises:
+    InvalidTreeNodeError: If the tree contains a node of a kind
+      that is not expected to show up.
+  '''
+
+  Examples:
+
+    (The behavior of a node-handler is somewhat nontrivial, so covering
+    two very common cases via examples is in order.)
+
+    This node-handler would allow descending into (nested)
+    instances of `list` (but not subclass instances thereof):
+
+    ```def list_node_handler(revtuple_path, obj):
+         ''' ... '''
+         if type(obj) is list:
+           return list, enumerate(obj)
+         else:
+           return None
+    ```
+
+    This node-handler would allow descending into (nested) mappings,
+    which upon rebuilding would get turned into `dict` instances:
+
+    ```def mapping_node_handler(revtuple_path, obj):
+         ''' ... '''
+         if isinstance(obj, collections.abc.Mapping):
+           # For generic mappings, we cannot rely on key- and item-iteration
+           # being guaranteed to use identical iteration-order.
+           items = list(obj.items())
+           keys = [kv[0] for kv in items]
+           return (lambda values: dict(zip(keys, values))), items
+         else:
+           return None
+    ```
+
+    A dict/mapping node-handler can of course rename keys, add or remove
+    entries, make decisions based on the item-path, or map a dict to
+    an associative list, etc.
+
+## Further Design Notes
+
+The `pytree_map` function requests the leaf-transform and node-handler
+to be side-effect-free functions. This is both required to leave
+implementation-side flexibility, and also follows the general LISP
+recommendation to not abuse mapping (which should be a pure
+data-transformation) for imperative data processing. Overall, if
+a need for more general "nested datastructures" processing becomes
+pressing, it is for the better if this leads to a proper articulation
+of the specific needs, to be addressed with appropriate design, rather
+than abuse of functional data-transforms becoming "a bad idiom
+that turned into established practice".
+
+"""
+
+import collections.abc
+import immutabledict
+
+import numpy
+
+from typing import Any, Callable, Iterable, Iterator, TypeVar
+
+
+T = TypeVar('T')
+U = TypeVar('U')
+
+KT = TypeVar('KT')
+NT = TypeVar('NT')
+
+
+## Type of the reverse-order-keys-to-root path.
+# (This code actually illustrates why https://xkcd.com/2483/ is very misguided.)
+RevTuplePath = tuple
+
+## Type of the `leaf_transform` function-argument used for tree-iteration.
+#
+# This would be the correct type we would have to specify here but cannot,
+# since the design of Python's static typing at the time of this writing
+# is too broken for that:
+#
+#   type LeafTransformFunc[L, R] = Callable[[RevTuplePath, L], R]
+#
+# Instead, we have to settle for...:
+LeafTransformFunc = Callable[[RevTuplePath, Any], Any]
+
+
+## Type of the `tree_node_handler` function-argument used for
+## tree-iteration and tree-mapping.
+#
+# Again, this is the correct type we would have to put here but cannot:
+#
+# type NodeHandlerFunc[KT] = (
+#   Callable[[NT],
+#            None | tuple[Callable[[Iterable[tuple[KT, NT]]], NT],
+#                         Iterator[tuple[KT, NT]]]])
+#
+# ...so, we have to instead settle for:
+NodeHandlerFunc = (
+  Callable[[RevTuplePath, NT],
+           None | tuple[Callable[[Iterable[tuple[Any, NT]]], NT],
+                        Iterator[tuple[Any, NT]]]])
+
+
+Predicate = Callable[[object], bool]
+
+
+class InvalidTreeNodeError(ValueError):
+  """Encountered a tree-node of invalid type."""
+
+
+def linearize_revtuple_path(
+    revtuple_path: RevTuplePath,
+    present_as: Callable[[Iterator[T]], U] = tuple) -> U:
+  """Translates a revtuple path to (typically) linear form.
+
+  With default `present_as`, this will map a path of the form
+  `(key_{N}, (key_{N-1}, ..., (root, ())))` into a tuple
+  (root, ..., key_{N-1}, key_{N}).
+
+  Args:
+    revtuple_path: A linked-list-as-recursive-pairs
+      reverse-order tuple-representation of the path.
+      Path-root is `()`, and node-key `x` relative to
+      earlier path `p` is represented as `(x, p)`.
+    present_as: Callable that consumes an iterator over
+      path-pieces - with the deepest-nesting level coming last -
+      turning it into a linearized path. Defaults to `tuple`.
+
+  Returns:
+    Linearized presentation of all the node-keys in the
+    recursive-path in order, deepest-down path component coming last.
+  """
+  pieces = []
+  todo = revtuple_path
+  while todo:
+    node, todo = todo
+    pieces.append(node)
+  return present_as(reversed(pieces))
+
+
+# This function itself has type `NodeHandlerFunc`, but Python does not
+# allow us to here simply type-annotate it like this. We cannot even
+# introduce an abbreviation for the complicated output-type,
+# since that would have to be parametric in node-type `NT` (and `KT`).
+def everything_is_a_leaf_node_handler(
+    revtuple_path: tuple,
+    node : NT) -> (
+    None | tuple[Callable[[Iterable[tuple[Any, NT]]], NT],
+                 Iterator[tuple[Any, NT]]]):
+  """Processes a tree-node as required by pytree-iteration and -mapping.
+
+  Interface and signature are in alignment with the requirements for a
+  "node handler" function explained in the module-docstring.
+
+  Args:
+    revtuple_path: the path-to-root for this node.
+    node: a tree-node.
+
+  Returns:
+    `None`, i.e. classifying any kind of node as a leaf-node.
+  """
+  del revtuple_path, node  # Unused.
+  return None
+
+
+def leaf_summary(path: RevTuplePath, x: object):
+  """Produces a human-readable summary-string for a leaf-node.
+
+  Args:
+    path: revtuple representation of the path-to-root.
+    x: The leaf-value.
+  """
+  del path  # Ignored here.
+  tx = type(x)
+  mod = tx.__module__
+  modname = mod if isinstance(mod, str) else mod.__name__
+  type_str = f'{modname}.{tx.__qualname__}'
+  repr_str = repr(x)
+  repr_abbrev = repr_str if len(repr_str) < 40 else repr_str[:40] + ' ...'
+  # On str, int, float, etc. `{type_str}(repr(x))` would actually still be
+  # a (non-literal) Python-expression that would evaluate to the original value.
+  # However, we make no promises beyond "human-readable".
+  return f'{type_str}({repr_abbrev})'
+
+
+# With respect to static type annotations, the limitations of Python's
+# approach to static typing really become prominently visible here.
+#
+# Different arguments have type-parameters, but since there is no way
+# to have parametric abbreviations such as `LeafTransformFunc[L, R]`,
+# the only way we would have available to express relations between
+# type-parameters would be to substitute in the not-abbreviated form of
+# `NodeHandlerFunc` and `LeafTransformFunc`, giving us something monstrous.
+# We instead here settle for "we cannot express that `tree` must
+# have the same type as the input-type to `tree_node_handler` and use `Any`,
+# and likewise for leaf_transform and the output.
+def pytree_leaf_iter(
+    tree: Any,
+    leaf_transform: LeafTransformFunc,
+    node_handler: NodeHandlerFunc = everything_is_a_leaf_node_handler,
+  ) -> Iterator[Any]:
+  # ...actual return type would be `Iterator[{what leaf_transform returns}]`.
+  """Iterates over the leaves of a tree.
+
+  Args:
+    tree: The tree to iterate over.
+    leaf_transform: A callable `f` that will get applied
+      as `f(revtuple_path, leaf)`, where `revtuple_path`
+      is the revtuple representation of the path to the
+      leaf from the root.
+    node_handler: A "node handler" (see module docstring)
+      that processes nodes encountered during iterative traversal.
+
+  Yields:
+    Value of `leaf_transform(p, x)`, where `x` is the current leaf
+    and `p` is its revtuple-path to the root.
+  """
+  # Note: Exit points for the code below are in non-obvious places
+  # and hence marked with " # ***EXIT***".
+  #
+  # Doing iteration properly is slightly nontrivial.
+  # One may be tempted to go for a very simple recursive implementation
+  # (with an extra pre-final `path` argument to `pytree_iter`):
+  #
+  # maybe_substructure = node_handler(path, tree)
+  # if maybe_substructure is None:
+  #   # We are looking at a leaf-node.
+  #   yield leaf_transform(path, tree)
+  # else:
+  #   _, contents_iter = maybe_substructure
+  #   for k, v in contents_iter:
+  #     yield from pytree_iter(v, leaf_transform, (k, path), node_handler)
+  #
+  # That, however, would be flawed, since there is no a priori reason
+  # why a pytree may not be a very deeply nested structure - such as a
+  # long linked list. That would then risk raising `RecursionError`,
+  # and since Python by design(!) does not perform tail call elimination
+  # or any other kind of advanced CPS transforms, there is no recursive
+  # solution here. So, to do this properly, we have to do this iteratively.
+  #
+  # We are facing an annoying situation here: If `tree` itself is a leaf,
+  # we have two options: (a) wrapping it up in a one-node tree
+  # and processing that, or (b) special-casing "root is a leaf".
+  # Option (b) leads to some mild node-processing code-duplication
+  # for a single node (the root).
+  # Option (a) requires having special cases for node-processing that
+  # get looked at for every tree node. We go with option (b) here.
+  maybe_substructure = node_handler((), tree)
+  if maybe_substructure is None:
+    # The tree itself is a leaf.
+    yield leaf_transform((), tree)
+    return  # ***EXIT***
+  # Otherwise, we are looking at a tree.
+  _, contents_iter = maybe_substructure
+  current_revtuple_path = ()
+  work_to_do = [contents_iter]
+  # Otherwise-unreachable sentinel for reliably identifying
+  # iterator-exhaustion without using exceptions:
+  sentinel = object()
+  while True:
+    current_iter = work_to_do[-1]
+    maybe_next_item = next(current_iter, sentinel)
+    if maybe_next_item is sentinel:
+      # We are done at this level.
+      work_to_do.pop()
+      if not work_to_do: return  # ***EXIT***
+      current_revtuple_path = current_revtuple_path[1]
+    else:
+      path_piece, subtree = maybe_next_item
+      extended_revtuple_path = (path_piece, current_revtuple_path)
+      maybe_subtree_substructure = node_handler(extended_revtuple_path, subtree)
+      if maybe_subtree_substructure is None:  # Case: subtree is a leaf.
+        yield leaf_transform(extended_revtuple_path, subtree)
+      else:  # Case: subtree is a tree.
+        current_revtuple_path = (path_piece, current_revtuple_path)
+        _, items_iter = maybe_subtree_substructure
+        work_to_do.append(items_iter)
+
+
+# The current design approach here would be appropriate for
+# applying leaf-transforms while retaining the structure of the tree -
+# which closely corresponds to e.g. a (a -> b) -> (Tree a -> Tree b) functor.
+#
+# It is not entirely clear whether this is the abstraction that we should
+# consider as being appropriately generic to flesh out explicitly - rather
+# than starting from a more general approach of which this then is a special
+# case. Some background: https://ncatlab.org/nlab/show/recursion+scheme
+#
+# On the other hand, there is a lot of flexibility via whatever
+# node-rebuilder a node-handler produces - this can do quite some reshaping
+# of a tree, including dropping or duplicating nodes.
+def pytree_map(
+    tree: Any,
+    leaf_transform,
+    node_handler: NodeHandlerFunc = everything_is_a_leaf_node_handler,
+  ):
+  """Maps a (potentially nested) Python value to another such value.
+
+  Args:
+    tree: The Python-object to be mapped.
+    leaf_transform: A callable `f` that will get applied
+      as `f(revtuple_path, leaf)`, where `revtuple_path`
+      is the revtuple representation of the path to the
+      leaf from the root. Must be side effect free.
+    node_handler: A "node handler" (see module docstring)
+      that processes nodes encountered during iterative traversal.
+      Must be side effect free.
+
+  Returns:
+    The outcome of translating `tree`.
+  """
+  # Note: Exit points for the code below are in non-obvious places
+  # and hence marked with " # ***EXIT***".
+  #
+  # Otherwise-inaccessible sentinel object, for reliably identifying
+  # missing-values via identity-check against sentinel lookup-default.
+  sentinel = object()
+  # Code structure mostly follows pytree_leaf_iter.
+  maybe_substructure = node_handler((), tree)
+  if maybe_substructure is None:
+    return leaf_transform((), tree)  # ***EXIT***
+  rebuilder, items_iter = maybe_substructure
+  current_revtuple_path = ()
+  # Per-level, we have a triplet of:
+  # (rebuilder, remaining_items_to_iterate_over, processed).
+  parts_for_assembly = [(rebuilder, items_iter, [])]
+  while True:
+    this_rebuilder, this_items_iter, this_done_pieces = parts_for_assembly[-1]
+    maybe_next_item = next(this_items_iter, sentinel)
+    if maybe_next_item is sentinel:
+      # We are done with all the items for this level.
+      parts_for_assembly.pop()
+      built_iter = this_rebuilder(this_done_pieces)
+      if not parts_for_assembly:  # No outer structure, so at-top-level.
+        return built_iter  # ***EXIT***
+      else:  # We have outer structure.
+        parts_for_assembly[-1][-1].append(built_iter)
+        current_revtuple_path = current_revtuple_path[1]
+        continue  # ...with next is-the-final-item-complete-check.
+    else:
+      # More constituents of the current item.
+      path_piece, subtree_item = maybe_next_item
+      extended_revtuple_path = (path_piece, current_revtuple_path)
+      maybe_subtree_substructure = node_handler(
+          extended_revtuple_path,
+          subtree_item)
+      if maybe_subtree_substructure is None:
+        this_done_pieces.append(
+            leaf_transform(extended_revtuple_path, subtree_item))
+      else:
+        # We have a subtree.
+        subtree_rebuilder, subtree_items_iter = maybe_subtree_substructure
+        current_revtuple_path = (path_piece,
+                                 current_revtuple_path)
+        parts_for_assembly.append(
+          (subtree_rebuilder, subtree_items_iter, []))
+
+
+def deep_freeze(
+    tree,
+    *,
+    is_mapping : Predicate = lambda x: isinstance(x, collections.abc.Mapping),
+    is_set : Predicate = lambda x: isinstance(x, collections.abc.Set),
+    is_sequence : Predicate = lambda x: isinstance(x, (list, tuple)),
+    leaf_fn: Callable[[Any], Any] = lambda x: x,
+    ):
+  """Recursively freezes Set/Mapping/List/Tuple structures.
+
+  Args:
+    tree: The potentially deeply-nested object to deep-freeze.
+    is_mapping: Callable that decides whether a sub-object is a mapping.
+      Defaults to an `isinstance()` check for `collections.abc.Mapping`.
+    is_set: Callable that decides whether a sub-object is a set.
+      Defaults to an `isinstance()` check for `collections.abc.Set`.
+    is_sequence: Callable that decides whether a sub-object is a sequence.
+      Defaults to a check for being a `tuple` or `list` instance.
+    leaf_fn: Function to use for translating non-mapping/set/sequence
+      instances.
+
+  Returns:
+    Translated-to-deeply-immutable form of `tree`.
+  """
+  idict = immutabledict.immutabledict
+  def freeze_node_handler(path, x):
+    if is_set(x):
+      return frozenset, ((None, y) for y in x)
+    if is_mapping(x):
+      # Mappings already have hashable, so
+      # (should-be-)deeply-immutable keys.
+      # Hence, we only need to deep-freeze the values.
+      #
+      # Note that non-`dict` mappings might not guarantee
+      # to respect iteration-order, so we have to be careful here:
+      items = list(x.items())
+      keys = [kv[0] for kv in items]
+      values = [kv[1] for kv in items]
+      return ((lambda ys: idict(zip(keys, ys))),
+              iter(items))
+    if is_sequence(x):
+      return tuple, enumerate(iter(x))
+    # Otherwise, this should not be traversed.
+    return None
+  def leaf_transform(revtuple_path, value):
+    del revtuple_path  # Unused.
+    return leaf_fn(value)
+  return pytree_map(tree, leaf_transform, freeze_node_handler)
diff --git a/compression/python/pytree/pytree_transforms_test.py b/compression/python/pytree/pytree_transforms_test.py
new file mode 100644
index 0000000..fdaec71
--- /dev/null
+++ b/compression/python/pytree/pytree_transforms_test.py
@@ -0,0 +1,168 @@
+"""Basic tests for 'algebraic data type based pytree' transformations."""
+
+
+import collections.abc
+import sys
+import unittest
+
+import pytree_transforms
+
+
+def _get_deep_pytree(packaging_fn, bottom, depth):
+  current = bottom
+  for n in reversed(range(depth)):
+    current = packaging_fn(n, current)
+  return current
+
+
+def _dict_node_handler(p, d):
+  del p  # Unused.
+  if isinstance(d, dict):
+    keys = d.keys()
+    newdict = lambda vals: dict(zip(keys, vals))
+    return (newdict, iter(d.items()))
+  else:
+    return None
+
+
+class PyTreeTest(unittest.TestCase):
+  """Basic correctness validation tests for PyTree transformations."""
+
+  def test_linearize_revtuple_path(self):
+    """Tests guarantees given by `linearize_revtuple_path`."""
+    linearize_revtuple_path = pytree_transforms.linearize_revtuple_path
+    with self.subTest(guarantee='empty'):
+      self.assertEqual(linearize_revtuple_path(()), ())
+    with self.subTest(guarantee='typical'):
+      self.assertEqual(linearize_revtuple_path((30, (20, (10, ())))),
+                       (10, 20, 30))
+    with self.subTest(guarantee='present_as'):
+      self.assertEqual(
+          linearize_revtuple_path(
+              (30, (20, (10, ()))), present_as=list),
+          [10, 20, 30])
+
+  def test_everything_is_a_leaf_node_handler(self):
+    """Tests guarantees given by `everything_is_a_leaf_node_handler`."""
+    everything_is_a_leaf_node_handler = (
+        pytree_transforms.everything_is_a_leaf_node_handler)
+    self.assertEqual(everything_is_a_leaf_node_handler((), 'abc'),
+                     None)
+    self.assertEqual(everything_is_a_leaf_node_handler(('b', ()),
+                                                       dict(a=3)),
+                     None)
+
+  def test_leaf_summary(self):
+    """Tests guarantees given by `leaf_summary`."""
+    # Since the docstring only guarantees "a human-readable presentation",
+    # we can and should only do loose checks.
+    thing = (5678, 9531)
+    summary = pytree_transforms.leaf_summary(('key', ()), thing)
+    self.assertIsInstance(summary, str)
+    self.assertIn(str(thing[0]), summary)
+    self.assertIn(str(thing[1]), summary)
+
+  def test_pytree_leaf_iter(self):
+    """Tests guarantees given by `pytree_leaf_iter`."""
+    pytree_leaf_iter = pytree_transforms.pytree_leaf_iter
+    def leaf_transform(path, leaf):
+      return repr(leaf) if path and path[0].startswith('R') else leaf
+    with self.subTest(guarantee='returns_iterator'):
+      result = pytree_leaf_iter(7, leaf_transform, _dict_node_handler)
+      self.assertIsInstance(result, collections.abc.Iterator)
+    with self.subTest(guarantee='totally_empty'):
+      result = list(pytree_leaf_iter({}, leaf_transform, _dict_node_handler))
+      self.assertEqual(result, [])
+    with self.subTest(guarantee='no_leaves'):
+      result = list(pytree_leaf_iter(dict(a={}),
+                                     leaf_transform, _dict_node_handler))
+      self.assertEqual(result, [])
+    with self.subTest(guarantee='is_leaf'):
+      result = list(pytree_leaf_iter(777, leaf_transform, _dict_node_handler))
+      self.assertEqual(result, [777])
+    with self.subTest(guarantee='generic'):
+      result = list(pytree_leaf_iter(
+          dict(n0=dict(n01=dict(n012=1002,
+                                n013=1003,
+                                Rn014=1004,
+                                ),
+                       n02=1005),
+               n5=1006),
+          leaf_transform, _dict_node_handler))
+      self.assertEqual(result, [1002, 1003, '1004', 1005, 1006])
+    with self.subTest(guarantee='with_keys'):
+      result = list(pytree_leaf_iter(
+          dict(n0=dict(n01=dict(n012=1002,
+                                n013=1003)),
+               n1=1004),
+          lambda p, s: (pytree_transforms.linearize_revtuple_path(p), s),
+          _dict_node_handler))
+      self.assertEqual(result,
+                       [(('n0', 'n01', 'n012'), 1002),
+                        (('n0', 'n01', 'n013'), 1003),
+                        (('n1',), 1004)])
+
+  def test_pytree_map(self):
+    """Tests guarantees given by `pytree_map`."""
+    pytree_map = pytree_transforms.pytree_map
+    leaf_transform = lambda p, s: repr(s)
+    tree1 = dict(t0=dict(t10=1001,
+                     t11=dict(t110=1002,
+                              t111=1003),
+                     t12=dict(t120=1004,
+                              t121=1005,
+                              t122=1006)),
+             t1=1007)
+    with self.subTest(guarantee='no_leaves'):
+      result = pytree_map(dict(a={}),
+                          leaf_transform,
+                          _dict_node_handler)
+      self.assertEqual(result, dict(a={}))
+    with self.subTest(guarantee='is_leaf'):
+      result = pytree_map(777, leaf_transform, _dict_node_handler)
+      self.assertEqual(result, '777')
+    with self.subTest(guarantee='generic'):
+      result = pytree_map(tree1, leaf_transform, _dict_node_handler)
+      self.assertEqual(result['t0']['t10'], '1001')
+
+  def test_deeply_nested(self):
+    """Tests correct behavior on deeply-nested data structures."""
+    pytree_leaf_iter = pytree_transforms.pytree_leaf_iter
+    pytree_map = pytree_transforms.pytree_map
+    #
+    depth = max(10**5, sys.getrecursionlimit() + 100)
+    deep_tree = _get_deep_pytree(lambda n, t: {n: t},
+                                 'leaf', depth)
+    with self.subTest(function='pytree_leaf_iter'):
+      leaves = list(pytree_leaf_iter(deep_tree,
+                                     lambda p, s: s.upper(),
+                                     _dict_node_handler))
+      self.assertEqual(leaves, ['LEAF'])
+    with self.subTest(function='pytree_map'):
+      mapped_deep_tree = pytree_map(deep_tree,
+                                    lambda p, s: s,
+                                    _dict_node_handler)
+      self.assertIsInstance(mapped_deep_tree, dict)
+    with self.subTest(function='combined'):
+      leaves = list(
+          pytree_leaf_iter(
+              pytree_map(deep_tree,
+                         lambda p, s: s.capitalize(),
+                         _dict_node_handler),
+              lambda p, s: s + s,
+              _dict_node_handler))
+      self.assertEqual(leaves, ['LeafLeaf'])
+
+  def test_deep_freeze(self):
+    """Tests guarantees given by `deep_freeze`."""
+    frozen = pytree_transforms.deep_freeze(
+        dict(a=[1001, 1002, dict(b=(1003, [1004, {1005, 1006}]))]))
+    self.assertIsInstance(frozen, collections.abc.Mapping)
+    self.assertNotIsInstance(frozen, collections.abc.MutableMapping)
+    self.assertIsInstance(frozen['a'], tuple)
+    # `frozen` is hashable, and hashes to an integer.
+    self.assertIsInstance(hash(frozen), int)
+
+
+if __name__ == '__main__':
+  unittest.main()
diff --git a/compression/python/pytree/requirements.txt b/compression/python/pytree/requirements.txt
new file mode 100644
index 0000000..90c3f39
--- /dev/null
+++ b/compression/python/pytree/requirements.txt
@@ -0,0 +1,4 @@
+immutabledict>=4.2.0
+numpy>=1.26.4
+orbax-checkpoint>=0.0.0
+