module type EltSig = 
sig
   type elt
   val compare : elt -> elt -> int
end

module type SetSig = 
sig
   type elt
   type t
   val create : unit -> t
   val mem : elt -> t -> bool
   val insert : elt -> t -> t
   val delete : elt -> t -> t
end

module MakeSplaySet (Elt : EltSig) = 
struct
   (* Same type of elements *)
   type elt = Elt.elt

   (* Splay trees *)
   type tree = 
      Leaf
    | Node of tree * elt * tree

   (* Have to use a reference cell to save result of splay *)
   type t = tree ref

   (* Splay operation *)
   let rec splay x = function
      Leaf -> Leaf
    | Node (left, y, right) -> 
         ... 

   (* Membership *)
   let mem x s = 
      let s' = splay x !s in
         let found = (* check if x is the root *) in
            s := s';
            found

   ...
end