Ltac InvBooleans :=
match goal with
| [ H: _ && _ = true |- _ ] =>
destruct (andb_prop _ _ H); clear H; InvBooleans
| [ H: _ || _ = false |- _ ] =>
destruct (orb_false_elim _ _ H); clear H; InvBooleans
| [ H: proj_sumbool ?x = true |- _ ] =>
generalize (proj_sumbool_true _ H); clear H; intro; InvBooleans
| _ => idtac
end.
Section DECIDABLE_EQUALITY.
Variable A: Type.
Variable dec_eq: forall (x y: A), {x=y} + {x<>y}.
Variable B: Type.

Lemma dec_eq_sym:
forall (x y: A) (ifso ifnot: B),
(if dec_eq x y then ifso else ifnot) =
(if dec_eq y x then ifso else ifnot).