title: How to fix using 'andalso' and ',' in Erlang's guard tests
body: <p>In the book Learn You Some Erlang for Great Good, it's said "the best strategy is often to mix them as necessary" about 'andalso', 'orelse', ',' and ';'.
  So, I try to coding like this, but it's compiled failure.</p>
  
  <pre><code>right_age(X) when (X&gt;=18,X=&lt;60) orelse (X=:=9) -&gt; true;
  right_age(_) -&gt; false.
  </code></pre>
  
  <p>Please give me a right example, thanks.</p>