@Override
    @CheckForNull
    public E lower(@ParametricNullness E e) {
      return delegate.lower(e);
    }

    @Override
    @CheckForNull
    public E floor(@ParametricNullness E e) {
      return delegate.floor(e);
    }

    @Override
    @CheckForNull
    public E ceiling(@ParametricNullness E e) {
      return delegate.ceiling(e);
    }

    @Override

          descendingSet = dS;
          return dS;
        }
        return descendingSet;
      }
    }

    @Override
    @CheckForNull
    public E floor(E e) {
      synchronized (mutex) {
        return delegate().floor(e);
      }
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {