push(stack, newAccum);
        }
        push(stack, collector.supplier().get());
        while (stack.size() > 1) {
          A right = pop(stack);
          A left = pop(stack);
          push(stack, collector.combiner().apply(left, right));
        }
        return pop(stack);
      }

      <E extends @Nullable Object> void push(List<E> stack, E value) {