docs: public <R> MapOperator<T, R> map(MapFunction<T, R> mapper) {
  		if (mapper == null) {
  			throw new NullPointerException("Map function must not be null.");
  		}
  
  		String callLocation = Utils.getCallLocationName();
  		TypeInformation<R> resultType = TypeExtractor.getMapReturnTypes(mapper, getType(), callLocation, true);
  		return new MapOperator<>(this, resultType, clean(mapper), callLocation);
  	}