ORELSE
op Tactical.ORELSE : tactic * tactic -> tactic
Applies first tactic, and if it fails, applies the second instead.
If T1 and T2 are tactics, T1 ORELSE T2 is a tactic which applies
T1 to a goal, and if it fails, applies T2 to the goal instead.
Failure
The application of ORELSE to a pair of tactics never fails. The
resulting tactic fails if both T1 and T2 fail when applied to the
relevant goal.