reverted to join_bodies/join_proofs based on fold_body_thms to regain performance (escpecially of HOL-Proofs) -- see also aa9c1e9ef2ce and 4e2abb045eac;
structure Nat =
struct
datatype nat = Suc of nat | Zero_nat;
end; (*struct Nat*)
structure Codegen =
struct
type 'a null = {null : 'a};
fun null (A_:'a null) = #null A_;
fun head A_ (x :: xs) = x
| head A_ [] = null A_;
val null_option : 'a option = NONE;
fun null_optiona () = {null = null_option} : ('a option) null;
val dummy : Nat.nat option =
head (null_optiona ()) [SOME (Nat.Suc Nat.Zero_nat), NONE];
end; (*struct Codegen*)