docs: @Override
  	public boolean validate(Graph<K, VV, EV> graph) throws Exception {
  		DataSet<Tuple1<K>> edgeIds = graph.getEdges()
  				.flatMap(new MapEdgeIds<>()).distinct();
  		DataSet<K> invalidIds = graph.getVertices().coGroup(edgeIds).where(0)
  				.equalTo(0).with(new GroupInvalidIds<>()).first(1);
  
  		return invalidIds.map(new KToTupleMap<>()).count() == 0;
  	}