Prove
(DA)
with no assumptions.
We prove (DA) by natural deduction.
We prove (DA) in both directions.
Direction from left to right:
Direction from right to left:
![[Graphics:indexgr539.gif]](indexgr539.gif)
![[Graphics:indexgr388.gif]](indexgr388.gif)
![[Graphics:indexgr538.gif]](indexgr538.gif)