title: Adding a tuple to a set does not work
body: <pre><code>scala&gt; val set = scala.collection.mutable.Set[(Int, Int)]()
  set: scala.collection.mutable.Set[(Int, Int)] = Set()
  
  scala&gt; set += (3, 4)
  &lt;console&gt;:9: error: type mismatch;
   found   : Int(3)
   required: (Int, Int)
                set += (3, 4)
                    ^
  
  scala&gt; set += Tuple2(3, 4)
  res5: set.type = Set((3,4))
  </code></pre>
  
  <p>Adding <code>(3, 4)</code> does not work - why ?</p>
  
  <p>Normally, <code>(3, 4)</code> also represents a tuple with two elements. </p>