code: public boolean enforceNode(int x, ICause cause) throws ContradictionException {
  		assert cause != null;
  		assert (x >= 0 && x < n);
  		if (UB.getNodes().contains(x)) {
  			if (LB.addNode(x)) {
  				if (reactOnModification) {
  					delta.add(x, GraphDelta.NE, cause);
  				}
  				GraphEventType e = GraphEventType.ADD_NODE;
  				notifyPropagators(e, cause);
  				return true;
  			}
  			return false;
  		}
  		this.contradiction(cause, "enforce node which is not in the domain");
  		return true;
  	}
docs: Enforce the node x to belong to any solution
  Adds x to the lower bound graph
  
  @param x     node's index
  @param cause algorithm which is related to the modification
  @return true iff the enforcing has an effect