path_to() returns the recursive path to the
first example matching the is_fn conditions,
or a list of paths to all matching items.
get_item() returns the item at the given path. If index is specified, get_item() will return that
item in the flattened version of items.
get_items() returns the items at the given paths as a LaTeX2 object. If index is specified, get_items() will return those
items in the flattened version of items.
set_item() replaces the item at the given path, and returns the modified version of items.
insert_values() inserts the values before
the item mentioned in path (or after if requested),
and returns the modified version of items.
get_container() returns the item
containing the given path
get_which() returns the index
of the item within its container.