We prove
by the deduction rule.
We assume
and show
.
From
and
we obtain by inverse modus ponens
![[Graphics:submitted.nbgr27.gif]](submitted.nbgr27.gif)
by the deduction rule. by the deduction rule.![[Graphics:submitted.nbgr28.gif]](submitted.nbgr28.gif)
![[Graphics:submitted.nbgr29.gif]](submitted.nbgr29.gif)
![[Graphics:submitted.nbgr30.gif]](submitted.nbgr30.gif)
![[Graphics:submitted.nbgr31.gif]](submitted.nbgr31.gif)
.![[Graphics:submitted.nbgr41.gif]](submitted.nbgr41.gif)
and ![[Graphics:submitted.nbgr42.gif]](submitted.nbgr42.gif)
we obtain by inverse modus ponens