title: How to specify multiple possible types (type union) for array in MSON?
body: <p>I'm struggling a bit to describe what I need exactly but I spent some time browsing the MSON spec and didn't find anything.</p>
  
  <p>This example is a bit contrived but hopefully the intention is clear.</p>
  
  <pre><code># Animal (object)
  
  This is a base class for all animals. It provides some common structure.
  
  ## Properties
   ... some properties
  
  # Bird (Animal)
  
  ## Properties
   ... some properties
  
  # Fish (Animal)
  
  ## Properties
   ... some properties
  
  # Farm (Object)
  
  ## Properties
   + animals (array[Animal], fixed-type)
  </code></pre>
  
  <p>I want to express that the Farm can contain all kinds of animals but NOT the base class. Is there some way to express that in MSON/JSON Schema so that it validates? Effectively I want to have a choice or type union of Bird and Fish. I don't mind writing out the classes explicitly.</p>
  
  <p>Thanks so much.</p>