code: public DataStreamSource<Long> generateSequence(long from, long to) {
  		if (from > to) {
  			throw new IllegalArgumentException("Start of sequence must not be greater than the end");
  		}
  		return addSource(new StatefulSequenceSource(from, to), "Sequence Source");
  	}
docs: Creates a new data stream that contains a sequence of numbers. This is a parallel source,
  if you manually set the parallelism to {@code 1}
  (using {@link org.apache.flink.streaming.api.datastream.SingleOutputStreamOperator#setParallelism(int)})
  the generated sequence of elements is in order.
  
  @param from
  The number to start at (inclusive)
  @param to
  The number to stop at (inclusive)
  @return A data stream, containing all number in the [from, to] interval