title: python all possible pairs of 2 list elements, and getting the index of that pair
body: <p>let's say I have two lists:</p>
  
  <pre><code>a = list(1,2,3)
  b = list(4,5,6)
  </code></pre>
  
  <p>So I can have 9 pairs of these list members:</p>
  
  <pre><code>(1,4)
  (1,5)
  (1,6)
  
  (2,4)
  (2,5)
  (2,6)
  
  (3,4)
  (3,5)
  (3,6)
  </code></pre>
  
  <p>Now, given two list members like above, can I find out the pair's index?
  Like (1,4) from above would be the 1st pair.</p>