title: "Invalid expression in guard, case is not allowed in guards" error
body: <p>The following code is deemed an error by <code>mix</code>:</p>
  
  <pre><code>case test do
        ...
        t when !is_list(t) -&gt; false
        ...
  end
  </code></pre>
  
  <p>The error is "Invalid expression in guard, case is not allowed in guards".</p>
  
  <p>However, if I remove the <code>!</code>, i.e. write</p>
  
  <pre><code>case test do
        ...
        t when is_list(t) -&gt; false
        ...
  end
  </code></pre>
  
  <p>no error is reported.</p>
  
  <p>This can't be right? <code>!</code> should just be the negation function.</p>