(** 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... *)

Lemma or_intro : forall A B : Prop, A -> A \/ B.