code: def postorder(self, funct, stopOn=None):
          
          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)
docs: 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.