structure ROOT = 

struct

structure IntDef = 

struct

datatype nat = Zero_nat | Succ_nat of nat;

fun less_nat Zero_nat (Succ_nat n) = true

  | less_nat na Zero_nat = false

  | less_nat (Succ_nat m) (Succ_nat nb) = less_nat m nb;

fun minus_nat (Succ_nat m) (Succ_nat n) = minus_nat m n

  | minus_nat Zero_nat na = Zero_nat

  | minus_nat y Zero_nat = y;

end; (*struct IntDef*)

structure Codegen = 

struct

fun pick ((k, v) :: xs) n =

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

  | pick (x :: xsa) na =

    let

      val (ka, va) = x;

    in

      (if IntDef.less_nat na ka then va

        else pick xsa (IntDef.minus_nat na ka))

    end;

end; (*struct Codegen*)

end; (*struct ROOT*)