// 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 oldVal = mapGet(c.fieldValues, field);
    
    if (value cast Concept) {
      if (oldVal instanceof Concept.Ref)
        // change existing reference
        ret ((Concept.Ref) oldVal).set(value);
      else {
        // overwrite non-reference value if any,
        // create new reference
        dynamicObject_setRawFieldValue(c, field, c.new Ref(value));
        c.change(); true;
      }
    } else {
      // value is not a concept
      
      // if it was a reference, cleanly delete it
      if (oldVal cast Concept.Ref) oldVal.unindexAndDrop();
    
      if (eq(oldVal, value)) false;
      
      if (isConceptList(value) && nempty(value/L)) {
        // TODO: clean-up etc
        dynamicObject_setRawFieldValue(c, field, c.new RefL(value/L));
        c.change(); true;
      }
      
      if (value == null) {
        // delete field
        dynamicObject_dropRawField(c, field);
      } else {
        // update or create field
        if (!isPersistable(value))
          fail("Can't persist: " + c + "." + field + " = "+ value);

        dynamicObject_setRawFieldValue(c, field, value);
      }
      c.change(); true;
    }
  } 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)) {
      bool isTransient = isTransient(f);
      if (!isTransient && !isPersistable(value))
        fail("Can't persist: " + c + "." + field + " = "+ value);
      f.set(c, value);
      if (!isTransient) c.change(); 
      true;
    }
  }
  false;
}