NULL_OK_LTTactical.NULL_OK_LT : list_tactic -> list_tactic
Makes a list-tactic succeed with no effect when applied to the empty goal list.
For any list-tactic ltac, the application
NULL_OK_LT ltac gives a new list-tactic which has the same
effect as ltac when applied to a non-empty goal list.
Applied to the empty goal list it succeeds with no effect.
The application of NULL_OK_LT to a list-tactic
ltac never fails. The resulting list-tactic fails if
applied to a non-empty goal list on which ltac fails.