def _eliminate_leafs(self, graph):
        """
        Eliminate leaf objects - that are objects not referencing any other
        objects in the list `graph`. Returns the list of objects without the
        objects identified as leafs.
        """
        result = []
        idset = set([id(x) for x in graph])
        for n in graph:
            refset = set([id(x) for x in get_referents(n)])
            if refset.intersection(idset):
                result.append(n)
        return result
Eliminate leaf objects - that are objects not referencing any other
        objects in the list `graph`. Returns the list of objects without the
        objects identified as leafs.