(* Example: simple recursive structures and signatures *) (* a simple recursive structure *) structure S =    rec (X:sig structure Odd : sig val test : int -> bool                               end           end)  (* forward declaration *)   struct structure Even = struct fun test 0 = true 				    | test n = X.Odd.test (n-1) (* forward reference *)			   end		  structure Odd = struct fun test 0 = false				   | test n = Even.test (n-1)			  end	          end;(* test *)val t1 = S.Even.test 100;val t2 = S.Odd.test 100; (* a simple recursive signature, specifying mutually recursive datatypes   that span module boundaries *) signature REC =     rec(X: sig structure Odd: sig type t end end)  (* the forward declaration *)    sig structure Even: sig datatype t = Zero                                        | Succ of X.Odd.t (* forward reference *)                        end	structure  Odd: sig datatype t = One                                         | Succ of Even.t end    end; (* the recursive signature can now be used to implement a    recursive structure T with mutually recursive datatypes    that span module boundaries *) structure T =     rec(X:REC) (* the forward declaration *)    struct 	structure Even = struct datatype t = datatype X.Even.t 				fun succ Zero = X.Odd.One 				  | succ E = X.Odd.Succ E			 end	structure Odd  = struct datatype t = datatype X.Odd.t				fun succ O = Even.Succ O			 end    end (* test *)val t3 = T.Even.Succ (T.Odd.Succ T.Even.Zero);