\documentclass{article}
  \usepackage{etoolbox}
  
  % --- preamble/postamble document hooks
  \AtEndPreamble{\typeout{atendpreamble}\def\secret{hello}}
  \AfterPreamble{\typeout{afterpreamble}\secret.\renewcommand{\secret}{world}}
  \AfterEndPreamble{\typeout{afterendpreamble}\secret.}
  \AfterEndDocument{never printed}
  
  \def\where{(outside)}
  \def\goinside{\def\where{(inside)}}
  
  \newenvironment{stuff}{Begin stuff.}{End stuff.}
  \AtBeginEnvironment{stuff}{\goinside}
  \AtBeginEnvironment{stuff}{At Begin stuff\where.}
  \AtEndEnvironment{stuff}{At End stuff\where.}
  \BeforeBeginEnvironment{stuff}{Before Begin stuff\where.}
  \AfterEndEnvironment{stuff}{After End stuff\where.}
  
  \AtBeginEnvironment{equation}{\goinside}
  \AtBeginEnvironment{equation}{At Begin equation\where.}
  \AtEndEnvironment{equation}{\textrm{ At End equation\where.}}
  \BeforeBeginEnvironment{equation}{Before Begin equation\where.}
  \AfterEndEnvironment{equation}{After End equation\where.}
  
  \AtBeginEnvironment{verbatim}{\goinside}
  \AtBeginEnvironment{verbatim}{Begin verbatim\where:\some \code.}
  \AtEndEnvironment{verbatim}{End verbatim\where: \some \code.}
  \BeforeBeginEnvironment{verbatim}{Before Begin verbatim\where.}
  \AfterEndEnvironment{verbatim}{After End verbatim\where.}
  
\begin{document}

\section{User commands}

% --- counters
\newcommand\basic{basic}
\robustify{\basic}
\protecting{\textit{\basic}}

\newcounter{countfour}
\defcounter{countfour}{2+2}
\the\value{countfour}

% --- lengths
%     source: https://tex.stackexchange.com/a/286171/78008
\newcommand{\etminof}[2]{%
	\ifdimgreater{#1}{#2}{%
		#2%
	}{%
		#1% Is smaller or equal
	}%
}
\newcommand{\etmaxof}[2]{%
	\ifdimgreater{#1}{#2}{%
		#1%
	}{%
		#2% Is greater or equal
	}%
}
\newlength{\foo}
\newlength{\foobar}
\deflength{\foo}{\etminof{1in}{2in}}
\deflength{\foobar}{\etmaxof{5in}{10in}}

\the\foo

\the\foobar

% --- 
\section{Hooks}

\begin{stuff}
	This is Stuff!
\end{stuff}

\begin{equation}
	a = b
\end{equation}

\def\some{Some }
\def\code{Code}
\begin{verbatim}
\some \code
\end{verbatim}

% --- 
\section{Author commands}
\subsection{Definitions}
\subsubsection{Macro definitions}
% --- overload test
\csdef{path-PB}{path of PB}
\csdef{path-UF}{path of UF}

\newrobustcmd\whoami{PB}
\newrobustcmd\mypath{\csuse{path-\whoami}}

\mypath

\renewrobustcmd\whoami{UF}

\mypath

\providerobustcmd\whoami{noop}

\mypath

% --- other shorthand utilities
\def\orange{orange}
\cslet{apple}\orange
\letcs\kiwi{apple}
\csletcs{fruit}{orange}
\apple = \kiwi = \fruit = orange

\def\deftestone{1}
\def\deftesttwo{1}
deftestone is \ifdefined\deftestone defined \else undefined\fi. After undef, \undef\deftestone
deftestone is \ifdefined\deftestone defined \else undefined\fi.

deftesttwo is \ifdefined\deftesttwo defined \else undefined\fi. After csundef, \csundef{deftesttwo}
deftesttwo is \ifdefined\deftesttwo defined \else undefined\fi.

\csmeaning{orange}

\subsubsection{Arithmetic definitions}
\numdef\arithone{2 + 4 * 8}
2 + 4 * 8 = \arithone

\bgroup\numgdef\arithtwo{3 + 5 * 7}\egroup
3 + 5 * 7 = \arithtwo

\bgroup\csnumgdef{ariththree}{4 + 6 * 8}\egroup
4 + 6 * 8 = \ariththree

\dimdef\dimone{1pt + 2em}
1pt + 2em = \dimone

\bgroup\dimgdef\dimtwo{3pt + 1em}\egroup
3pt + 1em = \dimtwo

\bgroup\csdimgdef{dimthree}{4pt + 0.5em}\egroup
4pt + 0.5em = \dimthree

\gluedef\glueone{10pt plus 2pt minus 3pt}
10pt plus 2pt minus 3pt = \glueone

\bgroup\csgluegdef{gluetwo}{4pt minus 1pt}\egroup
4pt minus 1pt = \gluetwo

\bgroup\csmugdef{muone}{5mu}\egroup
5mu = \muone
% ---
\subsection{Expansion control}
\def\lvlone{first}
\def\lvltwo{\lvlone}
\def\lvlthree{\lvltwo}
\edef\elvl{\lvlthree}
\edef\lvltest{\expandonce{\lvlthree}}
\edef\lvltesttwo{\csexpandonce{lvltwo}}
full expansion: \meaning\elvl

expand once: \meaning\lvltest

csexpandonce: \meaning\lvltesttwo

\subsection{Hook management}
% Source: https://www.dickimaw-books.com/latex/admin/html/hooks.shtml

\newcommand{\mymacro}{x}%
\mymacro;
\appto\mymacro{AB}%
\mymacro;
\appto\mymacro{YZ}%
\mymacro.

\renewcommand{\mymacro}{x}%
\newcommand{\myothermacro}{AB}%
\epreto\mymacro{\myothermacro}%
\renewcommand{\myothermacro}{YZ}%
\eappto\mymacro{\myothermacro}
\mymacro.

\renewcommand{\mymacro}{x}%
\newcommand{\mymacroB}{\mymacroC}%
\newcommand{\mymacroC}{\mymacroD}%
\newcommand{\mymacroD}{Z}%
\eappto\mymacro{\mymacroB}
\mymacro.

\newcommand{\premacro}{x}
\bgroup\csgpreto{premacro}{pre-}\egroup
\premacro.

\subsection{Patching}
\def\patchme{example -- here}
patching: \patchcmd\patchme{--}{patched}{ok}{fail}

\patchme

relax is \ifpatchable*{\relax}{}{not} patchable

begin is \ifpatchable*{\begin}{}{not} patchable

\def\paramacro#1#2{#1 and #2}
\paramacro{first}{second}

appending to paramacro: \apptocmd\paramacro{. Post}{ok}{fail}\newline
\paramacro{first}{second}

prepending to paramacro: \pretocmd\paramacro{Pre. }{ok}{fail}\newline
\paramacro{first}{second}

\subsection{Boolean Flags}

\newbool{boolvar}
boolvar is \ifbool{boolvar}{true}{false}.

\setbool{boolvar}{true}
set to true. boolvar is \ifbool{boolvar}{true}{false}.

\setbool{boolvar}{false}
set to false. boolvar is \ifbool{boolvar}{true}{false}.

\booltrue{boolvar}
set to true. boolvar is \notbool{boolvar}{false}{true}.

\boolfalse{boolvar}
set to false. boolvar is \notbool{boolvar}{false}{true}.

\newtoggle{ltxbool}
ltxbool is \iftoggle{ltxbool}{true}{false}

\settoggle{ltxbool}{true}
set to true. ltxbool is \iftoggle{ltxbool}{true}{false}

\settoggle{ltxbool}{false}
set to false. ltxbool is \iftoggle{ltxbool}{true}{false}

\toggletrue{ltxbool}
set to true. ltxbool is \nottoggle{ltxbool}{false}{true}

\togglefalse{ltxbool}
set to false. ltxbool is \nottoggle{ltxbool}{false}{true}

\subsection{Generic Tests}

noise is \ifcsdef{noise}{}{not} defined.

begin is \ifcsdef{begin}{}{not} defined.

relax is \ifcsdef{relax}{}{not} defined.

relax is ifundef: \ifundef{\relax}{true}{false}

relax is ifcsundef: \ifcsundef{relax}{true}{false}

nosuchmacro is \ifdefmacro{\nosuchmacro}{a macro}{not a macro}

relax is \ifdefmacro{\relax}{a macro}{not a macro}

begin is \ifdefmacro{\begin}{a macro}{not a macro}

nobeginning is \ifundef{\nobeginning}{un}{} defined.

begin is \ifundef{\begin}{un}{} defined.

relax is \ifdefparam{\relax}{}{not} defparam

begin is \ifdefparam{\begin}{}{not} defparam

apptocmd is \ifdefprotected{\apptocmd}{}{not} defprotected

ifdefprotected is \ifdefprotected{\ifdefprotected}{}{not} defprotected

relax is \ifdefprotected{\relax}{}{not} defprotected

begin is \ifdefprotected{\begin}{}{not} defprotected

prefix test begin: \ifcsprefix{begin}{true}{false}

prefix test noise: \ifcsprefix{noise}{true}{false}

prefix test newrobustcmd: \ifcsprefix{newrobustcmd}{true}{false}

prefix test newrobustcmd: \ifcsprefix{newrobustcmd}{true}{false}

ifcsltxprotect relax: \ifcsltxprotect{relax}{true}{false}

\DeclareRobustCommand{\myltxrobust}{test}
ifcsltxprotect myltxrobust: \ifcsltxprotect{myltxrobust}{true}{false}

\def\myempty{}
ifdefempty myempty: \ifdefempty\myempty{true}{false}

\def\myrelax{\relax}
\let\letrelax\relax
ifdefempty myrelax: \ifdefempty\myrelax{true}{false}

ifcsvoid myempty: \ifcsvoid{myempty}{true}{false}

ifcsvoid myrelax: \ifcsvoid{myrelax}{true}{false}

ifcsvoid letrelax: \ifcsvoid{letrelax}{true}{false}

ifcsvoid begin: \ifcsvoid{begin}{true}{false}

ifdefequal myempty-myrelax: \ifdefequal\myempty\myrelax{true}{false}

ifdefequal letrelax-myrelax: \ifdefequal\letrelax\myrelax{true}{false}

ifdefequal begin-begin: \ifdefequal\begin\begin{true}{false}

ifdefequal myempty-myempty: \ifdefequal\myempty\myempty{true}{false}

\long\edef\mymacro#1#2{\string&}
ifdefstring mymacro-ampersand: \ifdefstring{\mymacro}{&}{true}{false}

ifdefstring myrelax-relax: \ifdefstring{\myrelax}{\relax}{true}{false}

ifdefstring myempty-{}: \ifdefstring{\myempty}{}{true}{false}

ifdefstring basic-basic: \ifdefstring{\basic}{basic}{true}{false}

ifcsstrequal basic-basic \ifcsstrequal{basic}{basic}{true}{false}

ifcsstrequal relax-relax \ifcsstrequal{relax}{relax}{true}{false}

ifcsstrequal myempty-relax \ifcsstrequal{myempty}{relax}{true}{false}

\def\anotherempty{}
ifcsstrequal myempty-anotherempty \ifcsstrequal{myempty}{anotherempty}{true}{false}

% -- end of macro tests.
% -- start of counter and length tests:

ifdefcounter relax: \ifdefcounter{\relax}{true}{false}

\newcount\mycount
ifdefcounter mycount: \ifdefcounter{\mycount}{true}{false}

\mycount 4\relax
ifdefcounter mycount, after set: \ifdefcounter{\mycount}{true}{false}

\newcounter{latexcounter}
ifdefcounter latexcounter: \ifdefcounter{\latexcounter}{true}{false}

\stepcounter{latexcounter}
ifdefcounter latexcounter, after step: \ifdefcounter{\latexcounter}{true}{false}

\newskip\myskip
ifdefcounter myskip: \ifdefcounter{\myskip}{true}{false}

\newlength\mylen
ifdefcounter mylen: \ifdefcounter{\mylen}{true}{false}

\newdimen\mydimen
ifdefcounter mydimen: \ifdefcounter{\mydimen}{true}{false}

ifltxcounter relax: \ifltxcounter{relax}{true}{false}

ifltxcounter mycount: \ifltxcounter{mycount}{true}{false}

ifltxcounter latexcounter: \ifltxcounter{latexcounter}{true}{false}

ifdeflength relax: \ifdeflength{\relax}{true}{false}

ifdeflength mycount: \ifdeflength{\mycount}{true}{false}

ifdeflength myskip: \ifdeflength{\myskip}{true}{false}

ifdeflength mylen: \ifdeflength{\mylen}{true}{false}

ifdeflength mydimen: \ifdeflength{\mydimen}{true}{false}

ifdefdimen relax: \ifdefdimen{\relax}{true}{false}

ifdefdimen mycount: \ifdefdimen{\mycount}{true}{false}

ifdefdimen myskip: \ifdefdimen{\myskip}{true}{false}

ifdefdimen mylen: \ifdefdimen{\mylen}{true}{false}

ifdefdimen mydimen: \ifdefdimen{\mydimen}{true}{false}

% -- end of counter tests
% -- string tests

ifstrequal test 1 (false): \ifstrequal{my\relaxing}{\myrelaxing}{true}{false}

ifstrequal test 2 (true): \ifstrequal{my\relaxing}{my\relaxing}{true}{false}

ifstrequal test 3 (true): \ifstrequal{\myrelaxing}{\myrelaxing}{true}{false}

ifstrequal test 4 (true): \ifstrequal{simple}{simple}{true}{false}

ifstrequal test 5 (false): \ifstrequal{simple}{simpler}{true}{false}

ifstrempty test 1 (true): \ifstrempty{}{true}{false}

ifstrempty test 2 (false): \ifstrempty{\myempty}{true}{false}

ifstrempty test 3 (false): \ifstrempty{""}{true}{false}

ifstrempty test 4 (false): \ifstrempty{   }{true}{false}

ifblank test 1 (true): \ifblank{}{true}{false}

ifblank test 2 (false): \ifblank{\myempty}{true}{false}

ifblank test 3 (false): \ifblank{""}{true}{false}

ifblank test 4 (true): \ifblank{   }{true}{false}

notblank test 1 (false): \notblank{}{true}{false}

notblank test 2 (true): \notblank{\myempty}{true}{false}

notblank test 3 (true): \notblank{""}{true}{false}

notblank test 4 (false): \notblank{   }{true}{false}

% -- end of string tests
% -- arithmetic tests

ifnumcomp (false): \ifnumcomp{3}{>}{6}{true}{false}

ifnumcomp (true): \ifnumcomp{(7 + 5) / 2}{=}{6}{true}{false}

ifnumcomp (false): \ifnumcomp{(7+5) / 4}{>}{3*(12-10)}{true}{false}

\newcounter{countA}
\setcounter{countA}{6}
\newcounter{countB}
\setcounter{countB}{5}

ifnumcomp (true): \ifnumcomp{\value{countA} * \value{countB}/2}{=}{15}{true}{false}

ifnumcomp (true): \ifnumcomp{6/2}{=}{5/2}{true}{false}

% -- end arithemtic tests
% -- boolean tests
\newbool{boolA}
\newbool{boolB}

ifboolexpr (false): \ifboolexpr{bool {boolA}}{true}{false}

ifboolexpr (true): \ifboolexpr{not ( test {\ifcsdef{undefd}} or bool {boolA} )}
{true}{false}

\subsection{List Processing}

\noindent Itemize list:
\begin{itemize}
	\renewcommand*{\do}[1]{\item #1}
	      \docsvlist{item1, item2, {item3a, item3b}, item4}
\end{itemize}

\renewcommand*{\do}[1]{\indent * #1\newline}
\noindent Poor man's list:\newline
\docsvlist{item1, item2, {item3a, item3b}, item4}

\forcsvlist{\listadd\mylist}{itemA, itemB, itemC}
Dolistloop itemize:
\begin{itemize}
	\renewcommand*{\do}[1]{\item #1}
	      \dolistloop{\mylist}
\end{itemize}

\newcounter{itemcount}
Forlistloop itemize:
\begin{itemize}
	\forlistloop{\stepcounter{itemcount}\item}{\mylist}
	\item Total: \number\value{itemcount} items
\end{itemize}

\renewcommand*{\do}[1]{%
	\ifstrequal{#1}{itemB}
	{item found!\listbreak}
	{}}
dolistloop search: \dolistloop{\mylist}

\subsection{Miscellaneous Tools}

1995 = \rmntonum{mcmxcv}

\newcounter{rmcounter}
\setcounter{rmcounter}{\rmntonum{CXVI}}
116 = \the\value{rmcounter}

ifnumless (true): \ifnumless{\rmntonum{mcmxcviii}}{2000}{true}{false}

\end{document}