grammar.py 文件源码

def leftcorner_parents(self, cat):
        """
        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)
        """
        return self._leftcorner_parents.get(cat, set([cat]))