title: access elements of a list from a modified list
body: <p>Say I have a list:</p>
  
  <pre><code>[[0, 0], [0, 1], [1, 0], [0, 2], [1, 1], [2, 0], [0, 3], [1, 2], [2, 1], [3, 0]]
  </code></pre>
  
  <p>And I've produced another list from the one above, on the basis of some elements meeting a condition, lets say having a value equal to three:</p>
  
  <pre><code>[[0, 3], [3, 0]]
  </code></pre>
  
  <p>But now I want to access some elements from the bigger list, on the basis of some modification to my second list, lets say subtracting two from only those values equal to three in the second list. So I want to access those values in the first list taking the values [0,1] and [1,0] for the case of my second list here.
  How do I proceed? </p>