grammar.py 文件源码

def leftcorners(self, cat):
        """
        Return the set of all nonterminals that the given nonterminal
        can start with, including itself.

        This is the reflexive, transitive closure of the immediate
        leftcorner relation:  (A > B)  iff  (A -> B beta)

        :param cat: the parent of the leftcorners
        :type cat: Nonterminal
        :return: the set of all leftcorners
        :rtype: set(Nonterminal)
        """
        return self._leftcorners.get(cat, set([cat]))