code: @Nullable
      public <R> R reduceIndexed(int from, int step,
                                 @Nullable R identity,
                                 @NotNull IndexedBiFunction<? super R, ? super T, ? extends R> accumulator) {
          R result = identity;
          int index = from;
          while (iterator.hasNext()) {
              final T value = iterator.next();
              result = accumulator.apply(index, result, value);
              index += step;
          }
          return result;
      }
docs: Reduces the elements using provided identity value and
  the associative accumulation indexed function.
  
  <p>This is a terminal operation.
  
  <p>Example:
  <pre>
  from: 1
  step: 2
  identity: 10
  accumulator: (index, a, b) -&gt; index + a + b
  stream: [1, 2, 3, 4, 5]
  index:  [1, 3, 5, 7, 9]
  result: 10 + 2 + 5 + 8 + 11 + 14 = 50
  </pre>
  
  @param <R> the type of the result
  @param from  the initial value of the index (inclusive)
  @param step  the step of the index
  @param identity  the initial value
  @param accumulator  the accumulation function
  @return the result of the reduction
  @since 1.1.6