[dxbydt]

> you want .keySet instead of .keys, otherwise they won't be unique, right?

     scala> val p = Map(1->4,5->3)
     scala> p.keys
     res0: Iterable[Int] = Set(1, 5)
     scala> p.keySet
     res1: scala.collection.immutable.Set[Int] = Set(1, 5)
     scala> p.keys++p.keys
     res2: Iterable[Int] = Set(1, 5)
     scala> p.keySet++p.keySet
     res3: scala.collection.immutable.Set[Int] = Set(1, 5)