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