code: def p_type_list(self, p):
          '''type_list : type_list type_def
                       | empty'''
          if p[1] is None:
              p[0] = []
          else:
              p[1].append(p[2])
              p[0] = p[1f p_type_list(self, p):
          '''type_list : type_list type_def
                       | empty'''
          if p[1] is None:
              p[0] = []
          else:
              p[1].append(p[2])
              p[0] = p[1]
docs: type_list : type_list type_def
                       | empty