循环播放特定 Gmail 标签的邮件（而非线程）

我添加了具体的messages（而且不是全部thread) 到标签to_process通过以下步骤：

1. Turn Conversation ModeGmail 设置中关闭

2. 贴上标签to_process到特定消息

3. 当显示消息时，我可以确认只有特定的messages已添加。例如，同一线程中的另一条消息没有此标签。这很好。

现在我想循环处理来自 Google Apps 脚本的所有这些消息。但问题是API只能访问threads附加到某个标签：

var threads = GmailApp.search('label:to_process'); 
for (var i = 0; i < threads.length; i++) {
    // problem: here I cannot access to messages but only threads
}

or

var label = GmailApp.getUserLabelByName("to_process");
var threads = label.getThreads();
for (var i = 0; i < threads.length; i++) {
    // problem: here I cannot access to messages but only threads       
}

如何循环与标签关联的消息（而不是线程）？

解决方案的开始，但我不知道如何继续：

var threads = GmailApp.search('label:to_process'); 
for (var i = 0; i < threads.length; i++) {
    var messages = threads[i].getMessages();
    for (var j = 0; j < messages.length; j++) {
        var message = messages[j];

        // pseudo code here because getMessageLabels doesn't exist
        //if ("to_process" is in message.getMessageLabels()) {
        //}

    }
}

我会和高级 Gmail 服务 https://developers.google.com/apps-script/advanced/gmail和Gmail API 列表() https://developers.google.com/gmail/api/v1/reference/users/messages/list, then getMessageById() https://developers.google.com/apps-script/reference/gmail/gmail-app#getMessageById(String) :

function listMessages () {
  // Only return messages matching the specified query.
  var msgs = Gmail.Users.Messages.list('me', {'q':'label:to_process larger:5M'}).messages;

  // For each message - retrieve it by its id
  msgs.forEach(function (e){
    Logger.log("This email's subject is: %s", GmailApp.getMessageById(e.id).getSubject());
  });

}