¬¬(¬P∨P)
証明
https://scrapbox.io/files/660d3f487669750024403906.svg
code:proof.tikz(tex)
\usepackage{fitch}
\usepackage{amsmath}
\begin{document}
$\Large\begin{nd}
\open
\hypo {h} {\lnot(\lnot P\lor P)}
\open
\hypo {p} {P}
\have {npp} {\lnot P\lor P} \oi{p}
\have {b} {\bot} \ie{h,npp}
\close
\have {np} {\lnot P} \ii{p-b}
\have {npp} {\lnot P\lor P} \oi{np}
\have {b} {\bot} \ie{h,npp}
\close
\have {e} {\lnot\lnot(\lnot P\lor P)} \ii{h-b}
\end{nd}$
\end{document}