title: Filter array to show rows with a specific value in a specific column
body: <p>Let's say i have a multidimensional list l:</p>
  
  <pre><code>l = [['a', 1],['b', 2],['c', 3],['a', 4]]
  </code></pre>
  
  <p>and I want to return another list consisting only of the rows that has 'a' in their first list element:</p>
  
  <pre><code>m = [['a', 1],['a', 4]]
  </code></pre>
  
  <p>What's a good and efficient way of doing this?</p>