docs: public <R> FlatMapOperator<T, R> flatMap(FlatMapFunction<T, R> flatMapper) {
  		if (flatMapper == null) {
  			throw new NullPointerException("FlatMap function must not be null.");
  		}
  
  		String callLocation = Utils.getCallLocationName();
  		TypeInformation<R> resultType = TypeExtractor.getFlatMapReturnTypes(flatMapper, getType(), callLocation, true);
  		return new FlatMapOperator<>(this, resultType, clean(flatMapper), callLocation);
  	}