(** 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... *)

(** **** Exercise: 1 star (mult_eq_0)  *)
Lemma mult_eq_0 :
forall n m, n * m = 0 -> n = 0 \/ m = 0.