new method "elim_to_cases" provides ad-hoc conversion of obtain-style
elimination goals to a disjunction of existentials.
module ROOT =
struct
module Nat =
struct
type nat = Zero_nat | Suc of nat;;
end;; (*struct Nat*)
module Codegen =
struct
type 'a null = {null : 'a};;
let null _A = _A.null;;
let rec head _B = function x :: xs -> x
| [] -> null _B;;
let rec null_option = None;;
let null_optiona () = ({null = null_option} : ('b option) null);;
let rec dummy
= head (null_optiona ()) [Some (Nat.Suc Nat.Zero_nat); None];;
end;; (*struct Codegen*)
end;; (*struct ROOT*)