code: public <R> FlatMapOperator<T, R> flatMap(FlatMapFunction<T, R> flatMapper) {
  		if (flatMapper == null) {
  			throw new NullPointerException("FlatMap function must not be null.");
  		}
  		return new FlatMapOperator<T, R>(this, flatMapper);
  	}
docs: Applies a FlatMap transformation on a {@link DataSet}.<br/>
  The transformation calls a {@link FlatMapFunction} for each element of the DataSet.
  Each FlatMapFunction call can return any number of elements including none.
  
  @param flatMapper The FlatMapFunction that is called for each element of the DataSet.
  @return A FlatMapOperator that represents the transformed DataSet.
  
  @see FlatMapFunction
  @see FlatMapOperator
  @see DataSet