Discards a theorem already present in a goal's assumptions.
DESCRIBE
When applied to a theorem A' |- s and a goal, DISCARD_TAC
checks that s is simply T (true), or already exists (up
to alpha-conversion) in the assumption list of the goal. In
either case, the tactic has no effect. Otherwise, it fails.
A ?- t
======== DISCARD_TAC (A' |- s)
A ?- t
FAILURE
Fails if the above conditions are not met, i.e. the theorem's conclusion
is not T or already in the assumption list (up to alpha-conversion).