public static Batch <Element> entranceFor(Tile t) {
    final Batch <Element> batch = new Batch <Element> () ;
    for (Tile n : t.edgeAdjacent(tempT4)) {
      if (n == null || ! (n.owner() instanceof Boardable)) continue ;
      for (Boardable b : ((Boardable) n.owner()).canBoard(tempB4)) {
        if (b == t) batch.add((Element) n.owner()) ;
      }
    }
    return batch ;
  }