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) {
        stack.add(value);
      }

      <E extends @Nullable Object> E pop(List<E> stack) {
        return stack.remove(stack.size() - 1);

        if (node instanceof Element) {
            Element element = (Element) node;
            if (element.getTagName().equals("para") && stack.getFirst().getTagName().equals("para")) {
                pop();
                inPara = true;
            }
        }
        stack.getFirst().appendChild(node);
        if (inPara) {
            Element para = document.createElement("para");
            push(para);

    }

    @Override
    public void push(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.push(e);
    }

    @Override
    public E pop() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pop();
    }

    @Override
    public Iterator<E> descendingIterator() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.descendingIterator();
    }

  @Override
  @CheckForNull
  public E pollLast() {
    return delegate().pollLast();
  }

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness
  public E pop() {
    return delegate().pop();
  }

  @Override
  public void push(@ParametricNullness E e) {
    delegate().push(e);
  }

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness

    }

    @Override
    public void push(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.push(e);
    }

    @Override
    public E pop() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pop();
    }

    @Override
    public Iterator<E> descendingIterator() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.descendingIterator();
    }

          // An extra '?' or ',' after a child node indicates the end of all children of this node.
          idx++;
          break;
        }
      }
    }

    stack.pop();
    return idx - start;
  }

  private static CharSequence reverse(CharSequence s) {
    return new StringBuilder(s).reverse();
  }

  @Override
  @CheckForNull
  public E pollLast() {
    return delegate().pollLast();
  }

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness
  public E pop() {
    return delegate().pop();
  }

  @Override
  public void push(@ParametricNullness E e) {
    delegate().push(e);
  }

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness