module type OrderedType =sig..end
Map.Make.type 
val compare : t -> t -> intf 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 compare.