title: How to modify a json object using json11 library?
body: <p>I have a already <a href="https://github.com/dropbox/json11" rel="nofollow noreferrer">json11</a> object build:</p>
  
  <pre><code>Json my_json = Json::object {
          { "key1", "value1" },
          { "key2", false },
          { "key3", Json::array { 1, 2, 3 } },
      };
  </code></pre>
  
  <p>And I want to add a new value to key3 array like this:</p>
  
  <pre><code>my_json["keys3"].push_back(4);
  </code></pre>
  
  <p>How I can achieve that? I can't see anything to  modify objects (all operator to access values are const!)</p>