code: @Nullable
      public <R> R reduceIndexed(@Nullable R identity,
                                 @NotNull IndexedBiFunction<? super R, ? super T, ? extends R> accumulator) {
          return reduceIndexed(0, 1, identity, accumulator);
      }
docs: Reduces the elements using provided identity value and
  the associative accumulation indexed function.
  
  <p>This is a terminal operation.
  
  <p>Example:
  <pre>
  identity: 10
  accumulator: (index, a, b) -&gt; index + a + b
  stream: [1, 2, 3, 4, 5]
  index:  [0, 1, 2, 3, 4]
  result: 10 + 1 + 3 + 5 + 7 + 9 = 35
  </pre>
  
  @param <R> the type of the result
  @param identity  the initial value
  @param accumulator  the accumulation function
  @return the result of the reduction
  @since 1.1.6