'V.update_referrers(X)
Update the view V from the set X. X must be adaptable to NodeSet. V.rg is
updated so that in addition to its previos mapping, it will also contain
mappings for the elements of X to their referrers, from them to their
referrers and so on.'

def update_referrers(self, X):

    self.referrers(X)
