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.