A total ordering function over the keys.
This is a two-argument function f such that
f e1 e2 is zero if the keys e1 and e2 are equal,
f e1 e2 is strictly negative if e1 is smaller than e2,
and f e1 e2 is strictly positive if e1 is greater than e2.
Example: a suitable ordering function is the generic structural
comparison function Pervasives.compare.