# File lib/mail/parsers/rfc2822.rb, line 4623
    def _nt_msg_id_value
      start_index = index
      if node_cache[:msg_id_value].has_key?(index)
        cached = node_cache[:msg_id_value][index]
        if cached
          cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
          @index = cached.interval.end
        end
        return cached
      end

      i0, s0 = index, []
      r1 = _nt_id_left
      s0 << r1
      if r1
        if has_terminal?("@", false, index)
          r2 = instantiate_node(SyntaxNode,input, index...(index + 1))
          @index += 1
        else
          terminal_parse_failure("@")
          r2 = nil
        end
        s0 << r2
        if r2
          r3 = _nt_id_right
          s0 << r3
        end
      end
      if s0.last
        r0 = instantiate_node(SyntaxNode,input, i0...index, s0)
        r0.extend(MsgIdValue0)
      else
        @index = i0
        r0 = nil
      end

      node_cache[:msg_id_value][start_index] = r0

      r0
    end