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Prove
(False alternative) ![[Graphics:indexgr421.gif]](indexgr421.gif)
with no assumptions.
Proving (False alternative) by natural deduction fails.
The proof of (False alternative), by proving the first alternative negating the other(s), fails.
We assume
(False alternative.neg.1) ![[Graphics:indexgr422.gif]](indexgr422.gif)
.
We now show
(False alternative.C) ![[Graphics:indexgr423.gif]](indexgr423.gif)
.
We delete the (not false) assumption (False alternative.neg.1).
Sorry, proving (False alternative.C) fails.
![[Graphics:indexgr388.gif]](indexgr388.gif)
![[Graphics:indexgr420.gif]](indexgr420.gif)