new method "elim_to_cases" provides ad-hoc conversion of obtain-style
elimination goals to a disjunction of existentials.
structure ROOT =
struct
structure Nat =
struct
datatype nat = Zero_nat | Suc of nat;
fun plus_nat (Suc m) n = plus_nat m (Suc n)
| plus_nat Zero_nat n = n;
fun times_nat (Suc m) n = plus_nat n (times_nat m n)
| times_nat Zero_nat n = Zero_nat;
end; (*struct Nat*)
structure Codegen =
struct
fun fac (Nat.Suc n) = Nat.times_nat (Nat.Suc n) (fac n)
| fac Nat.Zero_nat = Nat.Suc Nat.Zero_nat;
end; (*struct Codegen*)
end; (*struct ROOT*)