'Return a dictionary that maps from the ``id`` of each feature
structure contained in ``self`` (including ``self``) to a
boolean value, indicating whether it is reentrant or not.'

def _find_reentrances(self, reentrances):

    if (id(self) in reentrances):
        reentrances[id(self)] = True
    else:
        reentrances[id(self)] = False
        for fval in self._values():
            if isinstance(fval, FeatStruct):
                fval._find_reentrances(reentrances)
    return reentrances
