[Tex/LaTex] Custom enumeration for axioms or hypothesis

#enumeratecross-referencinglists

This is a follow-up from : Custom Enumeration

I'd want to obtain something like :

Axioms list :
A1. First axiom 
A2. Second axiom
A2'. Equivalent version of the second axiom

with cross-reference, and also in a nice content-oriented manner. More specifically, I expect it to give convenient facility for reordering the items, and also it would be nice if it could generate all enumerations of the sort.
For example, I wouldn't expect to need a new command for :

Hypothesis list :
H1) First hypothesis
H2) Second
H2') Another version of the second

Redefining variables in the enumerate environment (locally or globally) is fine in this setting.

The best I have so far is :
(edited thanks to egreg's answer)

\documentclass{article}

\usepackage{enumitem}

\makeatletter
\def\myEnumCounter#1{\expandafter\@myEnumCounter\csname c@#1\endcsname}
\def\@myEnumCounter#1{\ifcase#1\or \or $'$\or $''$\fi}
\makeatother
\AddEnumerateCounter*{\myEnumCounter}{\@myEnumCounter}{3}

\newlist{axioms}{enumerate}{2}
\setlist[axioms,1]{label=\textbf{A\arabic{axiomsi}.}, ref=A\arabic{axiomsi}}
\setlist[axioms,2]{label=\textbf{A\arabic{axiomsi}\rlap{\myEnumCounter{axiomsii}}.},%
                   ref=A\arabic{axiomsi}\myEnumCounter{axiomsii},%
                   align=parleft,%
                   leftmargin=0em,%
                   itemsep=1.4ex,%
                   before={\stepcounter{axiomsi}}}

\begin{document}

\noindent Fail attempt %(useful to compare indentation and vertical separation): 
\begin{enumerate}[label=\textbf{A\arabic*.}]
  \item \label{item:A1} Axiom 1
  \item \label{item:A2} Axiom 2
  \item[\textbf{A2'}] \label{item:A2prime} Axiom 3
\end{enumerate}

\noindent "Almost there" attempt :
\begin{axioms}
  \item \label{item:B1} Axiom 1
  \item[]
  \begin{axioms}
    \item \label{item:B2} Axiom 2
    \item \label{item:B2prime} Axiom 2'
  \end{axioms}
\end{axioms}

\noindent References : \ref{item:A1}, \ref{item:A2} and \ref{item:A2prime} (last one doesn't work).

\noindent References : \ref{item:B1}, \ref{item:B2} and \ref{item:B2prime}.

\end{document}

The result is :
(non totally up-to-date)

Result

However, there are still a few issues :

  • I used the answer to nested enumeration, skipping items to avoid having an extra label in case I need to enter the second enumeration. However, the result somewhat fails to satisfy the "content-oriented" condition.Also, the situation here is a little different, because there is a scheme : I want a label if and only if the item isn't a new axioms environment. I guess there would be a way to do that with enumitem style key, but I can't figure how.
  • There is a 1mm offset of the sub-enumeration I can't suppress (I think I tried all the keys enumitem provides for horizontal spacing)
  • I had to add the itemsep=1.4ex to ensure the sub-enumeration has the same vertical spacing as the main one, but I suspect there is a better way to do this.

Best Answer

I'd simplify the whole thing:

\documentclass{article}

\usepackage{enumitem}

\newenvironment{axioms}
 {\enumerate[label=\textbf{A\arabic*.}, ref=A\arabic*]}
 {\endenumerate}
\makeatletter
\newcommand\varitem[1]{\item[\textbf{A\arabic{enumi}\rlap{$#1$}.}]%
  \edef\@currentlabel{A\arabic{enumi}{$#1$}}}
\makeatother


\begin{document}

\begin{axioms}
  \item \label{item:A1} Axiom 1
  \item \label{item:A2} Axiom 2
  \varitem{'} \label{item:A2prime} Axiom 2$'$
  \varitem{''} \label{item:A2dprime} Axiom 2$''$
  \item Axiom 3
  \varitem{'} Axiom 3$'$
\end{axioms}

\noindent References : \ref{item:A1}, \ref{item:A2} and \ref{item:A2prime}.

\end{document}

In my opinion, \rlap is better (and primes are surely better than apostrophes), but you can simply get rid of it.

If you want different letters it's better to redefine \varitem as part of the environment:

\providecommand{\varitem}{} % to keep LaTeX quiet
\makeatletter
\newenvironment{axioms}[1]
 {\renewcommand\varitem[1]{\item[\textbf{#1\arabic{enumi}\rlap{$##1$}.}]%
    \edef\@currentlabel{#1\arabic{enumi}{$##1$}}}%
  \enumerate[label=\textbf{#1\arabic*.}, ref=#1\arabic*]}
 {\endenumerate}
\makeatother

You'll have to pass the letter to the environment:

\begin{axioms}{A}
  \item \label{item:A1} Axiom 1
  \item \label{item:A2} Axiom 2
  \varitem{'} \label{item:A2prime} Axiom 2$'$
  \varitem{''} \label{item:A2dprime} Axiom 2$''$
  \item Axiom 3
  \varitem{'} Axiom 3$'$
\end{axioms}

Alternatively, you can define a "generic" environment and other environments based on it

\providecommand{\varitem}{} % to keep LaTeX quiet
\makeatletter
\newenvironment{statements}[1]
 {\renewcommand\varitem[1]{\item[\textbf{#1\arabic{enumi}\rlap{$##1$}.}]%
    \edef\@currentlabel{#1\arabic{enumi}{$##1$}}}%
  \enumerate[label=\textbf{#1\arabic*.}, ref=#1\arabic*]}
 {\endenumerate}
\makeatother

\newenvironment{axioms}{\statements{A}}{\endstatements}
\newenvironment{hypotheses}{\statements{H}}{\endstatements}

so that axioms will be used as before.

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