'Return the set of all nonterminals for which the given category
is a left corner. This is the inverse of the leftcorner relation.
:param cat: the suggested leftcorner
:type cat: Nonterminal
:return: the set of all parents to the leftcorner
:rtype: set(Nonterminal)'

def leftcorner_parents(self, cat):

    return self._leftcorner_parents.get(cat, set([cat]))
