sclass ConceptFieldIndex<A extends Concept, Val> implements IConceptIndex {
  Class<A> cc;
  S field;
  new HashMap<A, Val> objectToValue;
  new MultiMap<Val, A> valueToObject;
  
  *() {}
  *(Concepts concepts, Class<A> *cc, S *field) {
    concepts.addConceptIndex(this);
    for (A c : list(concepts, cc))
      update(c);
    concepts.addFieldIndex(cc, field, this);
  }
  
  public synchronized void update(Concept c) {
    Val newValue = cast cget(c, field);
    Val oldValue = objectToValue.get(c);
    if (newValue == null || newValue != oldValue) {
      valueToObject.remove(oldValue, (A) c);
      valueToObject.put(newValue, (A) c);
      objectToValue.put((A) c, newValue);
    }
  }
  
  public synchronized void remove(Concept c) {
    Val value = cast cget(c, field);
    objectToValue.remove(c);
    valueToObject.remove(value, (A) c);
  }
  
  synchronized A get(Val value) {
    ret first(valueToObject.get(value));
  }
  
  synchronized L<A> getAll(Val value) {
    ret valueToObject.get(value);
  }
}