| Fold of fold_kind * term * string list * term
(* base case pattern, recursive case bound names, recursive case pattern *)
| Default of term * term (* "some" case pattern, "none" case pattern *)
| Fold of fold_kind * term * string list * term
(* base case pattern, recursive case bound names, recursive case pattern *)
| Default of term * term (* "some" case pattern, "none" case pattern *)