structure Nat = 

 struct

 datatype nat = Suc of nat | Zero_nat;

 fun eq_nat (Suc a) Zero_nat = false

   | eq_nat Zero_nat (Suc a) = false

   | eq_nat (Suc nat) (Suc nat') = eq_nat nat nat'

   | eq_nat Zero_nat Zero_nat = true;

 end; (*struct Nat*)

 structure List = 

 struct

 fun null (x :: xs) = false

   | null [] = true;

 fun list_all2 p (x :: xs) (y :: ys) = p x y andalso list_all2 p xs ys

   | list_all2 p xs [] = null xs

   | list_all2 p [] ys = null ys;

 end; (*struct List*)

 structure Codegen = 

 struct

 datatype monotype = Mono of Nat.nat * monotype list;

 fun eq_monotype (Mono (tyco1, typargs1)) (Mono (tyco2, typargs2)) =

   Nat.eq_nat tyco1 tyco2 andalso

     List.list_all2 eq_monotype typargs1 typargs2;

 end; (*struct Codegen*)