// returns true if change

sbool _csetField(Concept c, S field, O value) ctex {

  Field f = setOpt_findField(c.getClass(), field);

  //print("cset: " + c.id + " " + field + " " + struct(value) + " " + f);

  if (value instanceof RC) value = c._concepts.getConcept((RC) value);

  value = deref(value);

  if (value instanceof S && l((S) value) >= concepts_internStringsLongerThan) value = intern((S) value);

  if (f == null) {

    // dynamic field (undeclared)

    assertIdentifier(field);

    O val2 = value instanceof Concept ? c.new Ref((Concept) value) : value;

    O oldVal = mapGet(c.fieldValues, field);

    if (oldVal instanceof Concept.Ref && val2 instanceof Concept.Ref)

      // change existing reference

      ((Concept.Ref) oldVal).set(((Concept.Ref) val2)!);

    else if (val2 == null) {

      // unindex & drop reference, drop dynamic field

      if (oldVal != null) {

        if (oldVal cast Concept.Ref)

          oldVal.unindexAndDrop();

        syncMapRemove(c.fieldValues, field);

      }

    } else

      // create new reference

      c.fieldValues = syncMapPut2_createLinkedHashMap(c.fieldValues, field, val2);

    c.change();

  } else if (isSubtypeOf(f.getType(), Concept.Ref.class)) {

    // Concept.Ref magic

    ((Concept.Ref) f.get(c)).set((Concept) derefRef(value));

    c.change(); true;

  } else if (isSubtypeOf(f.getType(), Concept.RefL.class)) {

    // Concept.RefL magic

    ((Concept.RefL) f.get(c)).replaceWithList(lmap derefRef((L) value));

    c.change(); true;

  } else {

    O old = f.get(c);

    if (neq(value, old)) {

      f.set(c, value);

      if (!isTransient(f)) c.change(); 

      true;

    }

  }

  false;

}