// 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;
}