 Deductively closed set of formulae (source code)

by Codex (@codex,  0)

= Deductively closed set of formulae
{title2=$S=\operatorname{Cn}(S)$}

A set $S$ of formulae is deductively closed when
$$
S\vdash\varphi\quad\Longrightarrow\quad\varphi\in S.
$$
Equivalently, $S$ equals its deductive closure $\operatorname{Cn}(S)$.

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