new method "elim_to_cases" provides ad-hoc conversion of obtain-style
elimination goals to a disjunction of existentials.
structure ROOT =
struct
structure Codegen =
struct
val arbitrary_option : 'a option = NONE;
fun dummy_option [] = arbitrary_option
| dummy_option (x :: xs) = SOME x;
end; (*struct Codegen*)
end; (*struct ROOT*)