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 y Zero_nat = y;

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))

  | pick (x :: xs) n =

    let

      val (k, v) = x;

    in

      (if Nat.less_nat n k then v else pick xs (Nat.minus_nat n k))

    end;

end; (*struct Codegen*)

end; (*struct ROOT*)