Emulation of Listing Six in http://www.cs.tut.fi/lintula/manual/modula3/modula-3/html/partial-rev/index.html

Association.scala

trait Association {
  type Item <: AbstractItem
  trait AbstractItem {
    def memberOf(): Group
  }
  type Group <: AbstractGroup
  trait AbstractGroup {
    def addItem(item: Item)
    def removeItem(item: Item)
    def items(): List[Item]
  }
}

AssociationModule.scala

object AssociationModule extends Association {
  type Item = ConcreteItem
  class ConcreteItem extends AbstractItem {
    var group: Group = _
    override def memberOf(): Group =  group
  }
  type Group = ConcreteGroup
  class ConcreteGroup extends AbstractGroup {
    var itemSet = Set[Item]()
    override def addItem(item: Item) = {
      itemSet += item
    }
    override def removeItem(item: Item) =  {
      itemSet -= item
    }
    override def items() = itemSet.toList
  }
}