code: @Override
  	public boolean removeArc(int x, int y, ICause cause) throws ContradictionException {
  		assert cause != null;
  		if (LB.arcExists(x, y)) {
  			this.contradiction(cause, "remove mandatory arc " + x + "->" + y);
  			return false;
  		}
  		if (UB.removeArc(x, y)) {
  			if (reactOnModification) {
  				delta.add(x, GraphDelta.AR_TAIL, cause);
  				delta.add(y, GraphDelta.AR_HEAD, cause);
  			}
  			GraphEventType e = GraphEventType.REMOVE_ARC;
  			notifyPropagators(e, cause);
  			return true;
  		}
  		return false;
  	}
docs: ***********************************************************************************