* subset: the numbers 1, 2 and 3 _must_ be in those three
* squares in some permutation, and hence we can deduce none of
* them can be in the fourth or fifth squares.)
+ *
+ * - Setwise positional elimination: if there is a subset of the
+ * unplaced numbers within a domain such that the union of all
+ * their possible positions has the same size as the subset
+ * itself, then all other numbers can be ruled out for those
+ * positions.
*/
/*