private final Object[] copy_slot_and_check( NonBlockingHashMap topmap, Object[] oldkvs, int idx, Object should_help ) {
      assert chm(oldkvs) == this;
      Object[] newkvs = _newkvs; // VOLATILE READ
      // We're only here because the caller saw a Prime, which implies a
      // table-copy is in progress.
      assert newkvs != null;
      if( copy_slot(topmap,idx,oldkvs,_newkvs) )   // Copy the desired slot
        copy_check_and_promote(topmap, oldkvs, 1); // Record the slot copied
      // Generically help along any copy (except if called recursively from a helper)
      return (should_help == null) ? newkvs : topmap.help_copy(newkvs);
    }