title: Find all indices of an element in a Groovy list
body: <p>I want to find positions of all elements of array with specific value. 
  Example:</p>
  
  <pre><code>def numbers = [1, 2, 3, 4, 5, 6, 5, 4, 6, 4, 9, 2];
  </code></pre>
  
  <p>I want to find positions of <code>4</code>, here position numbers are 3,7,9</p>
  
  <p>How to find it elegant with nice groovy collections methods?</p>