title: indexOf for a list of tuples with implicit element
body: <p>unfortunately I receive as index -1, whereas I would have liked to receive index 2
  Is there an alternative without going through my table for this problem?</p>
  
  <pre><code>val a = Array((1,2),(1,3),(2,1),(4,3));
  a.indexOf(Tuple2[Int,Int](2,_))
  </code></pre>