Refer to customized `enumerate` counter

Question

Background

I'm typesetting a list of conditions for a certain mathematical object. There are three conditions. Later on I wish to provide an equivalent condition for the third. Therefore, it would be nice to have this kind of numbering:

c) condition 1

b) condition 2

c) condition 3

while later on there would be

c') condition 3 (alternative)

My idea was to use a \label{c} for the original condition and write

\item[\ref{c}]\label{c-prime} ...

and it hoping it would be able to refer to c') this way. But \ref{c-prime} results in the number of the definition, not in the enumerate counter.

Solution

I've got a solution, based on a hard-coded starting point for the counter and fixed layout. Using enumitem I can write

\begin{enumerate}[label=\alph*'),ref=(\alph*'),start=3]

and everything works smoothly

Question

There are some drawbacks:

• I have to hand pick the starting point for the counter, rendering it impossible to automatically change the ordering;
• the formatting for labels and references is hard-coded, if I change the global format I still have to adapt it locally.

What can I do to make this more flexible?

I am aware this is a rather academic question as it already works, but I'm curious

Answer 1

The enumitem package gives you lots more—you can create enumerate-like list environments and set their format globally. Then you can use a counter to save the value of the current item to feed to start later on.

\documentclass{article}
\usepackage{lipsum}% just for blind text
\usepackage{enumitem}
\newlist{conditions}{enumerate}{1}
\newlist{altconditions}{enumerate}{1}
\setlist[conditions]{label=\alph*),ref=(\alph*)}
\setlist[altconditions]{label=\alph*$'$),ref=(\alph*$'$)}
\newcounter{favoritecondition}
\begin{document}

\lipsum[1]

\begin{conditions}
\item condition 1
\item condition 2
\item condition 3\label{c}\setcounter{favoritecondition}{\value{conditionsi}}
\item condition 4
\end{conditions}

\lipsum[2]

An alternative to condition \ref{c} is

\begin{altconditions}[start=\value{favoritecondition}]
\item\label{c-prime} condition 3 alternative 
\end{altconditions}

We leave as an exercise to the reader that \ref{c} and \ref{c-prime} are equivalent.
\end{document}