code: def p_arg_list(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: arg_list : ident_init_opt
               | arg_list COMMA ident_init_opt