code: public static <T> DataSet<LongValue> count(DataSet<T> input) {
  		return input
  			.map(new MapTo<>(new LongValue(1)))
  				.returns(LONG_VALUE_TYPE_INFO)
  				.name("Emit 1")
  			.reduce(new AddLongValue())
  				.name("Sum");
  	}
docs: Count the number of elements in a DataSet.
  
  @param input DataSet of elements to be counted
  @param <T> element type
  @return count