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

add class ring_div; generalize mod/diff/minus proofs for class ring_div

structure ROOT = 
struct

structure Codegen = 
struct

fun lookup ((k, v) :: xs) l =
  (if ((k : string) = l) then SOME v else lookup xs l)
  | lookup [] l = NONE;

end; (*struct Codegen*)

end; (*struct ROOT*)