- pairfrom(?Set, ?Element1, ?Element2, ?Residue) is semidet[private]
- is true when Set is a list, Element1 occurs in list, Element2 occurs
in list after Element1, and Residue is everything in Set bar the two
Elements. The point of this thing is to select_20_faster pairs of elements from
a set without selecting the same pair twice in different orders.