(* as it is dynamically inferred by simpl *)
Arguments minus !n / m.
simpl.
match goal with |- (match y with O => S x | S _ => _ end = 0) => idtac end.
Abort.
(* we avoid exposing a match *)
Arguments minus n m : simpl nomatch.
simpl.
match goal with |- (0 = 0) => idtac end.
Abort.
simpl.
match goal with |- (S x - y = 0) => idtac end.
Abort.
simpl.
match goal with |-(S x - (match y with O => _ | S _ => _ end) = 0) => idtac end.
Abort.
(* we unfold as soon as we have 1 args, but we avoid exposing a match *)
Arguments minus n / m : simpl nomatch.

Lemma foo : minus 0 = fun x => 0.