like having a game of play with a cart-horse, and expecting every
moment to be trampled under its feet, ran round the thistle
again; then the puppy began a series of short charges at the
stick, running a very little way forwards each time and a long
way back, and barking hoarsely all the while, till at last it sat
down a good way off, panting, with its tongue hanging out of its
mouth, and its great eyes half shut.

      return Ordering.from(forward).reverse();
    }

    @Override
    @ParametricNullness
    public K firstKey() {
      return forward().lastKey();
    }

    @Override
    @ParametricNullness
    public K lastKey() {
      return forward().firstKey();
    }

    @Override
    @CheckForNull

like having a game of play with a cart-horse, and expecting every
moment to be trampled under its feet, ran round the thistle
again; then the puppy began a series of short charges at the
stick, running a very little way forwards each time and a long
way back, and barking hoarsely all the while, till at last it sat
down a good way off, panting, with its tongue hanging out of its
mouth, and its great eyes half shut.