code: public static DataSet<Vertex<LongValue, NullValue>> vertexSequence(ExecutionEnvironment env, int parallelism, long vertexCount) {
  		Preconditions.checkArgument(vertexCount >= 0, "Vertex count must be non-negative");
  
  		if (vertexCount == 0) {
  			return env
  				.fromCollection(Collections.emptyList(), TypeInformation.of(new TypeHint<Vertex<LongValue, NullValue>>(){}))
  					.setParallelism(parallelism)
  					.name("Empty vertex set");
  		} else {
  			LongValueSequenceIterator iterator = new LongValueSequenceIterator(0, vertexCount - 1);
  
  			DataSource<LongValue> vertexLabels = env
  				.fromParallelCollection(iterator, LongValue.class)
  					.setParallelism(parallelism)
  					.name("Vertex indices");
  
  			return vertexLabels
  				.map(new CreateVertex())
  					.setParallelism(parallelism)
  					.name("Vertex sequence");
  		}
  	}
docs: Generates {@link Vertex Vertices} with sequential, numerical labels.
  
  @param env the Flink execution environment.
  @param parallelism operator parallelism
  @param vertexCount number of sequential vertex labels
  @return {@link DataSet} of sequentially labeled {@link Vertex vertices}