find_caption() returns a LaTeX2range object
for any caption
text, with an attribute extra holding the range of
associated macros and whitespace.
drop_caption() returns the items with
captions dropped as a LaTeX2 object. It has an attribute
named idx that is the idx argument with corresponding
elements dropped.
path_to_caption() returns a path
containing the location of the first caption
block within items. It has an attribute extra
containing a LaTeX2range object for the
associated macros and whitespace.