structure ROOT =
struct
structure Nat =
struct
datatype nat = Zero_nat | Suc of nat;
fun less_nat n (Suc m) = less_eq_nat n m
| less_nat n Zero_nat = false
and less_eq_nat (Suc n) m = less_nat n m
| less_eq_nat Zero_nat m = true;
fun minus_nat (Suc m) (Suc n) = minus_nat m n
| minus_nat Zero_nat n = Zero_nat
| minus_nat m Zero_nat = m;
end; (*struct Nat*)
structure Codegen =
struct
fun pick ((k, v) :: xs) n =
(if Nat.less_nat n k then v else pick xs (Nat.minus_nat n k));
end; (*struct Codegen*)
end; (*struct ROOT*)