code: def p_exprs(p):
      
      if len(p) == 2:
          p[0] = node.expr_list([p[1]])
      elif len(p) == 4:
          p[0] = p[1]
          p[0].append(p[3])
      else:
          assert (0)
      assert isinstance(p[0], node.expr_list)
docs: exprs : expr
            | exprs COMMA expr