sclass ConceptFieldIndex<A extends Concept, Val> extends ConceptFieldIndexBase<A, Val> {
  *(Class<A> cc, S field) { super(cc, field); }
  *(Concepts concepts, Class<A> cc, S field) { super(concepts, cc, field); }

  void init {
    valueToObject = new MultiSetMap;
  }
  
  void register {
    concepts.addFieldIndex(cc, field, this);
  }
}