title: How to represent sum/union types in in json schema
body: <p>I am trying to document an existing use of JSON using json-schema.  The system permits the following two possabilities for one of the object attributes.</p>
  
  <p>Either</p>
  
  <pre><code>{
      "tracking_number" : 123
  }
  </code></pre>
  
  <p>Or</p>
  
  <pre><code>{
      "tracking_number" : [ 123, 124, 125 ]
  }
  </code></pre>
  
  <p>How can I express this using json schema?</p>