public void accept(@ParametricNullness T t) {
        this.holder = t;
      }

      @Override
      public boolean tryAdvance(Consumer<? super R> action) {
        if (fromSpliterator.tryAdvance(this)) {
          try {
            // The cast is safe because tryAdvance puts a T into `holder`.
            action.accept(function.apply(uncheckedCastNullableTToT(holder), index++));
            return true;