private void clearArrow(Node x, Node y) {
final OrderedPair<Node> pair = new OrderedPair<Node>(x, y);
final Set<Arrow> lookupArrows = this.lookupArrows.get(pair);
if (lookupArrows != null) {
sortedArrows.removeAll(lookupArrows);
}
this.lookupArrows.remove(pair);
}
