val (--) : int ‑> int ‑> int ta -- b is the range of integers from a to b, both included,
in increasing order. It will therefore be empty if a > b.
val (--^) : int ‑> int ‑> int ta --^ b is the range of integers from b to a, both included,
in decreasing order (starts from a).
It will therefore be empty if a < b.