This is the proof I was provided in one direction.
I am confused as to why there is a necessity to introduce variables $s, d$. How does one make the leap there $\exists d$. Any proof clarification would be really appreciated!
2026-03-25 23:11:05.1774480265
$\Sigma \vDash \exists x a(x)$ if and only if $\Sigma \vDash (\neg \forall x( \neg a(x))).$
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