'Return a string representation of this feature structure.
:param reentrances: A dictionary that maps from the ``id`` of
each feature value in self, indicating whether that value
is reentrant or not.
:param reentrance_ids: A dictionary mapping from each ``id``
of a feature value to a unique identifier.  This is modified
by ``repr``: the first time a reentrant feature value is
displayed, an identifier is added to ``reentrance_ids`` for it.'

def _repr(self, reentrances, reentrance_ids):

    raise NotImplementedError()
