title: Is it valid to reduce on an empty set of sets?
body: <p>Shouldn't this work?</p>
  
  <pre><code>&gt; val setOfSets = Set[Set[String]]()    
  setOfSets: scala.collection.immutable.Set[Set[String]] = Set()
  
  &gt; setOfSets reduce (_ union _)
  java.lang.UnsupportedOperationException: empty.reduceLeft
    at scala.collection.TraversableOnce$class.reduceLeft(TraversableOnce.scala:152)
    [...]
  </code></pre>