If lines immidately follows a list item, without the list marker
at their begining, but with a offset matching the list item, then
these lines will be appended to that list item.
If a empty line is there between a list item and a new line with
some content, but without a list marker
* if the content offset is less than the last list item, then
unwind the lists before such a line.
* if the content offset is larger than the last list item, then
the line will be added as a new list item at the same level
as the last list item.
* if the content offset is same as the last list tiem, then
unwind the list by one level and then insert this line as a
new list item at this new unwound level.
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