forgot the sources :D

src/multisets-lib/Multiset.scala

+package scala.collection.immutable
+
+case class UnimplementedFeatureException() extends Exception
+
+object Multiset {
+  def empty[A] : Multiset[A] = throw UnimplementedFeatureException()
+  def apply[A](elems: A*) : Multiset[A] = throw UnimplementedFeatureException()
+}
+
+trait Multiset[A] {
+  def ** (that: Multiset[A]) : Multiset[A] = throw UnimplementedFeatureException()
+  def ++ (that: Multiset[A]) : Multiset[A] = throw UnimplementedFeatureException()
+  def +++ (that: Multiset[A]) : Multiset[A] = throw UnimplementedFeatureException()
+  def -- (that: Multiset[A]) : Multiset[A] = throw UnimplementedFeatureException()
+  def toSet : scala.collection.immutable.Set[A] = throw UnimplementedFeatureException()
+  def size : Int = throw UnimplementedFeatureException()
+}