code: @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;
  	}
docs: Checks that the edge set input contains valid vertex Ids, i.e. that they
  also exist in the vertex input set.
  
  @return a boolean stating whether a graph is valid
  with respect to its vertex ids.