Custom enumeration for axioms or hypothesis

Question

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)

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.

Answer 1

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.