code: public final TypeV putIfMatchUnlocked( Object key, Object newVal, Object oldVal ) {
      if( oldVal == null ) oldVal = TOMBSTONE;
      if( newVal == null ) newVal = TOMBSTONE;
      final TypeV res = (TypeV)putIfMatch( this, _kvs, key, newVal, oldVal );
      assert !(res instanceof Prime);
      //assert res != null;
      return res == TOMBSTONE ? null : res;
    }
docs: then the put inserted newVal, otherwise it failed.