code: public Graph<K, VV, EV> addVertex(final Vertex<K, VV> vertex) {
  		List<Vertex<K, VV>> newVertex = new ArrayList<>();
  		newVertex.add(vertex);
  
  		return addVertices(newVertex);
  	}
docs: Adds the input vertex to the graph. If the vertex already
  exists in the graph, it will not be added again.
  
  @param vertex the vertex to be added
  @return the new graph containing the existing vertices as well as the one just added