In addition to the normal counters for enumeration environments, which now count down, etaremune
uses a second counter called EM@itemctr
.
This counter just counts up like normal and it is used to determine how many items the environment has so that the right starting value can be used during the next run.
You can thus skip an item in an etaremune
environment by decreasing \@enumctr
(= enum<i+>
where <i+>
stands for an appropriate number of i
's) by one and increasing EM@itemctr
by one.
The macro \etaremuneskip
, which I define below, does precisely this (and it takes an optional argument in case you want to skip multiple items).
\documentclass{article}
\pagestyle{empty}
\usepackage{etaremune}
\makeatletter %% <- make @ usable in command names
\newcommand*\etaremuneskip[1][1]{%
\addtocounter{EM@itemctr}{#1}%
\addtocounter{\@enumctr}{-#1}%
}
\makeatother %% <- revert @
\begin{document}
\begin{etaremune}
\item the fourth\label{fourth}
\begin{etaremune}
\item the f-th
\item the e-th
\etaremuneskip[3]
\item the a-th
\end{etaremune}
\item the third \label{third}
\etaremuneskip
\item the first\label{first}
\end{etaremune}
\end{document}

(Apart from the fact that it is localised to the place where the item should otherwise be inserted, I don't think this has any advantages to the answer you posted yourself though.)