New gather and equation environments with a single counter

Question

So I've created a new equation-style environment with an associated counter to track a certain set of definitions I introduce in my document:

\newcounter{defcounter}
\newenvironment{defeq}{%
\refstepcounter{defcounter}
\renewcommand\theequation{D.\thedefcounter}
\begin{equation}}
{\end{equation}}

This produces the desired result: a new environment defeq just like equation, but with its own counter and its own label format (e.g., (D.1)).

But occasionally I'll have several definitions in a row and want to use a gather environment to avoid the ugly vertical spacing that results from consecutive equation environments. A slight modification of the above kind of works:

\newenvironment{defgather}{%
\refstepcounter{defcounter}
\renewcommand\theequation{D.\thedefcounter}
\gather}
{\endgather}

However, linebreaking with \\ doesn't seem to step the newly created counter. The following MWE illustrates the issue:

\documentclass{article}
\usepackage{amsmath}
    \newcounter{defcounter}
    \newenvironment{defeq}{%
    \refstepcounter{defcounter}
    \renewcommand\theequation{D.\thedefcounter}
    \begin{equation}}
    {\end{equation}}

    \newenvironment{defgather}{%
    \refstepcounter{defcounter}
    \renewcommand\theequation{D.\thedefcounter}
    \gather}
    {\endgather}
\begin{document}

\begin{defeq}
    1
\end{defeq}

Text text text text.

\begin{defgather}
    2 \\
    3
\end{defgather}

\end{document}

The lines containing "1" and "2" are appropriately labelled -- (D.1) and (D.2). However, the line containing "3" is labelled (D.2) indicating that the \\ hasn't stepped defcounter as the default gather does with the equation counter.

How can I fix my new gather environment so that \\ steps the new counter?

Answer 1

It's easiest to \let the defcounter to that of equation within your environment(s). That way defcounter is stepped whenever equation would be:

\documentclass{article}
\usepackage{amsmath}
\newcounter{defcounter}

\makeatletter
\newenvironment{defeq}{%
  \let\c@equation\c@defcounter% equation is equivalent to defcounter
  \renewcommand\theequation{D.\arabic{equation}}
  \begin{equation}}
  {\end{equation}}

\newenvironment{defgather}{%
  \let\c@equation\c@defcounter% equation is equivalent to defcounter
  \renewcommand\theequation{D.\arabic{equation}}
  \gather}
  {\endgather}
\makeatother

\begin{document}

\begin{equation}
  abc \label{eq:first}
\end{equation}

\begin{defeq}
  1 \label{deq:first}
\end{defeq}

There is~\eqref{eq:first}, \eqref{deq:first}, \eqref{deq:second},~\eqref{deq:third} 
and~\eqref{eq:second}.

\begin{defgather}
  2 \label{deq:second} \\
  3 \label{deq:third}
\end{defgather}

\begin{equation}
  abc \label{eq:second}
\end{equation}

\end{document}