docs: def p_x_commalist(self,t):
      """commalist : commalist ',' expression
                   | expression
      """
      if len(t) == 2: t[0] = CommaX([t[1]])
      elif len(t) == 4: t[0] = CommaX(t[1].children+[t[3]])
      else: raise NotImplementedError('unk_len',len(t)) # pragma: no cover