code: public static <T, R> StreamParametersMapFunction<T> map(int idx,
              Function<T, R> mapFunction) {
          if (idx < 0) {
              throw new IllegalArgumentException("idx can't be negative");
          }
          return new StreamParametersMapFunction<T>().andMap(idx, mapFunction);
      }
docs: Start building a Parameter Mapper function, with an initial converter.
  <p>
  Not specified index are considered transformed by identity function.
  
  @param idx
  The parameter index
  @param mapFunction
  the function to be applied
  @return the function on the parameter array
  @param <T>
  The input type for the function
  @param <R>
  the result type for the function