code: @Deprecated
  	public <R> SingleOutputStreamOperator<R> fold(R initialValue, FoldFunction<T, R> folder) {
  
  		TypeInformation<R> outType = TypeExtractor.getFoldReturnTypes(
  				clean(folder), getType(), Utils.getCallLocationName(), true);
  
  		return transform("Keyed Fold", outType, new StreamGroupedFold<>(clean(folder), initialValue));
  	}
docs: Applies a fold transformation on the grouped data stream grouped on by
  the given key position. The {@link FoldFunction} will receive input
  values based on the key value. Only input values with the same key will
  go to the same folder.
  
  @param folder
  The {@link FoldFunction} that will be called for every element
  of the input values with the same key.
  @param initialValue
  The initialValue passed to the folders for each key.
  @return The transformed DataStream.
  
  @deprecated will be removed in a future version