code: @NotNull
      public LongStream mapIndexed(int from, int step,
                                   @NotNull IndexedLongUnaryOperator mapper) {
          return new LongStream(params, new LongMapIndexed(
                  new PrimitiveIndexedIterator.OfLong(from, step, iterator),
                  mapper));
      }
docs: Returns a {@code LongStream} with elements that obtained
  by applying the given {@code IndexedLongUnaryOperator}.
  
  <p>This is an intermediate operation.
  
  <p>Example:
  <pre>
  from: -2
  step: 2
  mapper: (index, value) -&gt; (index * value)
  stream: [ 1, 2, 3,  4]
  index:  [-2, 0, 2,  4]
  result: [-2, 0, 6, 16]
  </pre>
  
  @param from  the initial value of the index (inclusive)
  @param step  the step of the index
  @param mapper  the mapper function used to apply to each element
  @return the new stream
  @since 1.2.1