title: Typescript how to define callback function?
body: <p>I have function like this:</p>
  
  <pre><code> .add({a: 1, b: 2}, function (msg, reply) {
      reply({z: msg.z})
    })
  </code></pre>
  
  <p>It tried something like this:</p>
  
  <pre><code>interface SenecaMethods {
      add: (patern: object, HERE SHOULD BE CALLBACK) =&gt; object;
  }
  </code></pre>
  
  <p>How can define type for this?</p>