impl_keep_tac
Tactic.impl_keep_tac : tactic
Implements a version of implication-left sequent calculus rule as tactic
Given a goal of the form A ?- ((p ==> q) ==> r), an application of
impl_tac will produce two sub-goals: A ?- p and A, p ?- (q ==> r).
This can be useful if p should be dealt with in isolation, when, say,
the tactics that solve p can't safely be applied to q and/or r.
This tactic differs from impl_tac in that it keeps p as an
assumption in the second sub-goal.
Failure
Fails if the goal is not an implication with another implication as its
antecdent. Note that for the purpose of this tactic, a negation ~p is
viewed as the implication p ==> F. This means that impl_tac will
succeed when applied to goals whose conclusions are ~p ==> q,
~(p ==> q) and ~~p.