title: Nested groupingBy for maps with an inner ImmutableList?
body: <p>I have something along these lines:</p>
  
  <pre><code>Map&lt;String, Map&lt;String, List&lt;SomeClass&gt;&gt;&gt; someMap = 
      .... (streams and filtering, etc.) 
      .collect(Collectors.groupingBy(
         x -&gt; getSomeComputedAttribute(x),
         Collectors.groupingBy(x -&gt; getSomeOtherComputedAttribute(x));
  </code></pre>
  
  <p>This, so far, is working fine - I'm getting the right values, but the problem is I need to turn the List into an ImmutableList. What is a simple way to do this? </p>