'V.referrers_gc(X) -> idset
Return the set of objects that directly refer to
any of the objects in the set X.
This differs from referrers in that it uses the
gc module\'s view of the referrers. This is more or less
valid depending on viewpoint.'

def referrers_gc(self, X):

    X = tuple(self.nodeset_adapt(X))
    return (self.idset(self.gc.get_referrers(*X)) - self.iso(X))
