module type OrderedType = sig .. end
Input signature of the functor 
Set.Make.
type t;
The type of the set elements.
let compare: (t, t) => int;
A 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.
          Example: a suitable ordering function is the generic structural
          comparison function 
Pervasives.compare.