title: Remove last element of array in Redux with immutability-helper without knowing array's length
body: <p>I am using <code>immutability-helper</code> (<a href="https://github.com/kolodny/immutability-helper" rel="nofollow noreferrer">https://github.com/kolodny/immutability-helper</a>) to update state in Redux actions. I have an array and I'm adding an element like this:</p>
  
  <p><code>update(state, { modalAlerts: { $push : [payload] } })</code></p>
  
  <p>It works great. However, I don't know how to remove the <strong>last</strong> element of the array using Immutability Helper <em>without knowing the size of the array</em>. What is the equivalent of something like <code>update(state, { $pop: modalAlerts } })</code> (it doesn't exist) to remove the last element?</p>