title: Does mobx-state-tree support nested models?
body: <p>I have a parent model and a state-child model, and I'd like to reference actions in the child model as such <code>item.pdf.download()</code>.</p>
  
  <p>I have created a workaround as such:</p>
  
  <pre><code>const pdf = types.model({ state: ... })
  const Item = types.model({ ... }).volatile(self =&gt; ({ pdf: pdf.create({ ... }) }))
  </code></pre>
  
  <p>But it doesn't look like the intended way,</p>
  
  <p>Is there a way to do nested module in mobx-state-tree?</p>