type tThe type of the elements in the set.
The chosen t must be infinite.
val compare : t -> t -> intA total ordering function over the set elements. This is a two-argument function f such that:
f e1 e2 is zero if the elements 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.