}

    @Override
    public Iterator<E> iterator() {
      return delegate().iterator(); // manually synchronized
    }

    @Override
    public Spliterator<E> spliterator() {
      synchronized (mutex) {
        return delegate().spliterator();
      }
    }

    @Override
    public Stream<E> stream() {
      synchronized (mutex) {
        return delegate().stream();
      }
    }

        final Supplier<String> pathSupplier = () -> {
            final boolean parallel = false;
            return StreamSupport.stream(((Iterable<ActivationFrame>) stk::descendingIterator).spliterator(), parallel)
                    .map(f -> f.location)
                    .collect(Collectors.joining("."));
        };
        final Supplier<InputLocation> locationSupplier = () -> {