def postorder(self, funct, stopOn=None):
        """ Iterates in postorder, calling the function with the current node.
        If stopOn is set to True or False, it will stop on true or false.
        """
        if stopOn is None:
            for i in range(len(self.children) - 1, -1, -1):
                self.children[i].postorder(funct)
        else:
            for i in range(len(self.children) - 1, -1, -1):
                if self.children[i].postorder(funct) == stopOn:
                    return stopOn
        return funct(self.symbol)
Iterates in postorder, calling the function with the current node.
        If stopOn is set to True or False, it will stop on true or false.