title: Reduce on tuples
body: <p>I have the following type: <code>val l:Seq[(List[Int],Set[Int])] = ...</code>  </p>
  
  <p>I am trying to reduce it as follows:  </p>
  
  <pre><code>l.reduceLeft {  
      case ((a1,b1),(a2,b2)) =&gt; if((b1 &amp; b2).nomEmpty) ((a1:::a2),(b1 | b2)) else (a1,b1) }
  </code></pre>
  
  <p>I get the following:  </p>
  
  <pre><code>error:Expression of type (List[Int],Set[Int]) doesn't conform to expected type B_ 
  </code></pre>
  
  <p>What am I doing wrong?</p>