I ended up doing too much manually. This was the first major project I took on in Pressbooks, and I eventually abandoned it because of higher priority projects.

PD did not use standard language for his proofs, and wrote his own style sheet.

I added this in the advanced tab of quicklatex

\usepackage{amsmath}

\usepackage{amsfonts}

\usepackage{amssymb}

\usepackage{mdwtab}

\usepackage{ifthen}

\usepackage{latexsym}

% Line height in proofs

\newlength{\fitchlineht}

\setlength{\fitchlineht}{1.5\baselineskip}

% Horizontal indent between proof levels

\newlength{\fitchindent}

\setlength{\fitchindent}{0.7em}

% Indent to comment

\newlength{\fitchcomind}

\setlength{\fitchcomind}{2em}

% Line number width

\newlength{\fitchnumwd}

\setlength{\fitchnumwd}{1em}

% Altered from mdwtab.sty: shorter vline, for start of subproof

\makeatletter

\newcommand\fvline[1][\arrayrulewidth]{\vrule@height.5\fitchlineht@width#1\relax}

\makeatother

% Ordinary vertical line

\newcommand{\fa}{\vline\hspace*{\fitchindent}}

% Vertical line, shorter: Use at start of (sub)proof

\newcommand{\fb}{\fvline\hspace*{\fitchindent}}

% Hypothesis

\newcommand{\fh}{\fvline%

\makebox[0pt][l]{{%

\raisebox{-1.4ex}[0pt][0pt]{\rule{1.5em}{\arrayrulewidth}}}}%

\hspace*{\fitchindent}}

% Hypothesis, with longer vert line: for >1 hypothesis

\newcommand{\fj}{\vline%

\makebox[0pt][l]{{%

\raisebox{-1.4ex}[0pt][0pt]{\rule{1.5em}{\arrayrulewidth}}}}%

\hspace*{\fitchindent}}

% Modal subproof: takes argument = operator

\newcommand{\fitchmodal}[1]{%

\makebox[0pt][r]{${}^{#1}$,}\fvline\hspace*{\fitchindent}}

\newcommand{\fn}{\fitchmodal{\Box}}% Box subproof

\newcommand{\fp}{\fitchmodal{\Diamond}}% Diamond subproof

% Modal subproof with hypothesis in first line (as in Fitch)

\newcommand{\fitchmodalh}[1]{%

\makebox[0pt][r]{${}^{#1}$,}%

\fvline%

\makebox[0pt][l]{{%

\raisebox{-1.4ex}[0pt][0pt]{\rule{1.5em}{\arrayrulewidth}}}}%

\hspace*{\fitchindent}}

% Rule: formula introduction marker. \fr with line, \fs without line

\newcommand{\fr}{%

\makebox[0pt][r]{${\rhd}$,,}\vline\hspace*{\fitchindent}}

\newcommand{\fs}{%

\makebox[0pt][r]{${\rhd}$,,}}

% Box around argument, like new variable in ql

\newcommand{\fw}[1]{\fbox{\footnotesize $#1$}}

%

\newcounter{fitchcounter}

\setcounter{fitchcounter}{0}

%To avoid starting from 1, \setboolean{resetfitchcounter}{false}

\newboolean{resetfitchcounter}

\setboolean{resetfitchcounter}{true}

%To avoid increasing numbers, \setboolean{increasefitchcounter}{false}

\newboolean{increasefitchcounter}

\setboolean{increasefitchcounter}{true}

%\formatfitchcounter can be altered if need be, though only once per proof

\newcommand{\formatfitchcounter}[1]{\scriptsize \arabic{#1}}

%Typeset the counter

\newcommand{\fitchcounter}{%

\ifthenelse{\boolean{increasefitchcounter}}{\addtocounter{fitchcounter}{1}}{}

\formatfitchcounter{fitchcounter}}

%A line with a special number – a tag, e.g. \ftag{\vdots}{}

\newcommand{\ftag}[2]{\multicolumn{1}%

{!{\makebox[\fitchnumwd][r]{#1}\hspace{\fitchindent}}Ml@{\hspace{\fitchcomind}}}%

{#2}}

\newenvironment{fitchnum}%

{\ifthenelse{\boolean{resetfitchcounter}}{\setcounter{fitchcounter}{0}}{}

\begin{tabular}{!{\makebox[\fitchnumwd][r]{\fitchcounter }\hspace{\fitchindent}}Ml@{\hspace{\fitchcomind}}l}}%

{\end{tabular}}

\newenvironment{fitchunum}%

{\begin{tabular}{!{\makebox[\fitchnumwd][r]{}\hspace{\fitchindent}}Ml@{\hspace{\fitchcomind}}l}}%

{\end{tabular}}

\newenvironment{fitch}{\renewcommand{\arraystretch}{1.5}

\begin{fitchnum}}{\end{fitchnum}}

\newenvironment{fitch*}{\renewcommand{\arraystretch}{1.5}

\begin{fitchunum}}{\end{fitchunum}}

% The following is useful for giving a numbered formula, then the proof.

\newenvironment{flem}[2]%

{\begin{eqnarray}

\label{#2}\

&\begin{fitch}}%

{\end{fitch}\notag\end{eqnarray}}

%To write comment field for two consecutive lines, with brace

\newcommand{\ftwocom}[1]{%

\parbox[t]{3cm}{

\raisebox{-.6\baselineskip}[\baselineskip][0pt]{%

$\left.

\begin{aligned}

,\ ,

\end{aligned}

\right}$\quad #1}

}}

Seeing your post has made me try to pick up this project again. Back when I first tried this, I did a lot of cutting and pasting and too much manual work. I am wondering now if I could export the LaTeX > EPUB or HTML and have an easier time loading into Pressbooks.