(** Conversely, to show that a disjunction holds, we need to show that one of its sides does. This is done via two tactics, [left] and [right].  As their names imply, the first one requires proving the left side of the disjunction, while the second requires proving its right side.  Here is a trivial use... *)

(* Specifications: *)
Lemma or_intro : forall A B : Prop, A -> A \/ B.
(** ... and a slightly more interesting example requiring both [left] and [right]: *)
Lemma zero_or_succ :
forall n : nat, n = 0 \/ n = S (pred n).
(** **** Exercise: 1 star (mult_eq_0)  *)
Lemma mult_eq_0 :
forall n m, n * m = 0 -> n = 0 \/ m = 0.