(** 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... *)

(** ... and a slightly more interesting example requiring both [left] and [right]: *)
Lemma zero_or_succ :
forall n : nat, n = 0 \/ n = S (pred n).