docs: public Object getGetterOrSetter(String name, int index, boolean isSetter)
      {
          if (name != null && index != 0)
              throw new IllegalArgumentException(name);
          Slot slot = slotMap.query(name, index);
          if (slot == null)
              return null;
          if (slot instanceof GetterSlot) {
              GetterSlot gslot = (GetterSlot)slot;
              Object result = isSetter ? gslot.setter : gslot.getter;
              return result != null ? result : Undefined.instance;
          }
          return Undefined.instance;
      }