title: google protocol buffers: how to define message contains ArrayList<ArrayList<String>> in proto file
body: <p>ArrayList corresponds to repeated string:</p>
  
  <pre><code>message m1 {
      repeated string mylist = 1;
  }
  </code></pre>
  
  <p>How to define ArrayList&lt; ArrayList&lt; String>     > in message?
  Thanks!</p>