reverted to join_bodies/join_proofs based on fold_body_thms to regain performance (escpecially of HOL-Proofs) -- see also aa9c1e9ef2ce and 4e2abb045eac;
structure Codegen =
struct
val arbitrary_option : 'a option = NONE;
fun dummy_option [] = arbitrary_option
| dummy_option (x :: xs) = SOME x;
end; (*struct Codegen*)