class SortedMultiMap[K,V] private (private val delegate: SortedMap[K, Set[V]])
                                  (implicit ord: Ordering[K])
      extends SortedMap[K, Set[V]]
      with SortedMapLike[K, Set[V], SortedMultiMap[K,V]]

override def +[B1 >: Set[V]](kv: (K, B1)) = new SortedMultiMap(delegate + kv)

override def +[B1 >: Set[V]](kv: (K, B1)): SortedMap[K, B1] = delegate + kv // general case
def +(kv: (K,Set[V])): SortedMultiMap[K, V] = new SortedMultiMap(delegate + kv) // specific case

override def +[B1 >: Set[V]](kv: (K, B1)) = new SortedMultiMap(delegate + kv)

override def +[B1 >: Set[V]](kv: (K, B1)): SortedMap[K, B1] = kv._2 match {
  // may need some massaging to persuade the compiler
  case v: Set[a] => new SortedMultiMap(delegate + (kv._1, v))
  case _ => delegate + kv
}