TRY_LT
Tactical.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.
Failure
The application of TRY_LT to a list-tactic never fails. The resulting
list-tactic never fails.