TRY_LTTactical.TRY_LT : (list_tactic -> list_tactic)
Makes a list-tactic have no effect rather than fail.
For any list-tactic ltac, the application
TRY_LT ltac gives a new list-tactic which has the same
effect as ltac if that succeeds, and otherwise has no
effect.
The application of TRY_LT to a list-tactic never fails.
The resulting list-tactic never fails.