title: Create function which take objects that declare an implicit writes
body: <p><em>This is a dumbed-down example of my problem, ignore the semantics of the question</em></p>
  
  <p>I have two sets of objects that each define an implicit <code>writes</code> function:</p>
  
  <pre><code>case class User(id: Int, name: String)                      // Defines an implicit writes
  case class Group(id: Int, membersCount: Int, adminID: Int)  // Defines an implicit writes
  </code></pre>
  
  <p>How can I create a function that accepts any parameter that defines an implicit writes function?  So that I can pass an instance of either object to that function:</p>
  
  <pre><code>def format(w: DefinesWrites): JsValue = {
     ...
  }
  </code></pre>