doc-src/IsarAdvanced/Codegen/Thy/examples/pick2.ML

metis_tac: proper context (ProofContext.init it *not* sufficient);
simplified method setup;

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*)