structure Nat = 

 struct

 datatype nat = Suc of nat | Zero_nat;

 fun less_nat m (Suc n) = less_eq_nat m n

   | less_nat n Zero_nat = false

 and less_eq_nat (Suc m) n = less_nat m n

   | less_eq_nat Zero_nat n = true;

 end; (*struct Nat*)

 structure Codegen = 

 struct

 fun in_interval (k, l) n =

   Nat.less_eq_nat k n andalso Nat.less_eq_nat n l;

 end; (*struct Codegen*)