code: @PublicEvolving
  	public AllWindowedStream<T, W> sideOutputLateData(OutputTag<T> outputTag) {
  		Preconditions.checkNotNull(outputTag, "Side output tag must not be null.");
  		this.lateDataOutputTag = input.getExecutionEnvironment().clean(outputTag);
  		return this;
  	}
docs: Send late arriving data to the side output identified by the given {@link OutputTag}. Data
  is considered late after the watermark has passed the end of the window plus the allowed
  lateness set using {@link #allowedLateness(Time)}.
  
  <p>You can get the stream of late data using
  {@link SingleOutputStreamOperator#getSideOutput(OutputTag)} on the
  {@link SingleOutputStreamOperator} resulting from the windowed operation
  with the same {@link OutputTag}.