Exercise 8.7 [nqueens-size-exercise]
Which of the following are valid (necessarily true) sentences?
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$(\exists x\ xx) {:\;{\Rightarrow}:\;}({\forall\,y\;\;} \exists z\ yz)$.
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${\forall\,x\;\;} P(x) \lor \lnot P(x)$.
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${\forall\,x\;\;} {Smart}(x) \lor (xx)$.
 
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