op 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.
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