CHANGED_TAC
Tactical.CHANGED_TAC : (tactic -> tactic)
Makes a tactic fail if it has no effect.
When applied to a tactic T, the tactical CHANGED_TAC gives a new
tactic which is the same as T if that has any effect, and otherwise
fails.
Failure
The application of CHANGED_TAC to a tactic never fails. The resulting
tactic fails if the basic tactic either fails or has no effect.