How to make = work like + in breqn?

Question

I want to write chains of equations like ugly formula = expanded = moving things around = simplified = nice formula and to have linebreaks in space-efficient places regardless of margin size and the number of columns. I feel like the formulas shouldn't be broken up so that the reader can parse them without moving their eyes all the time. I was looking around and stumbled upon breqn, which seems to be powerful enough but somehow not willing to fulfill my wishes. It linebreaks on any =, for example

\documentclass{article}
\usepackage{breqn}
\begin{document}
\begin{dmath*}
a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a
\end{dmath*}
\end{document}

will make a tower of =as. The =s can be substituted with \hiderel{=}s but then it never linebreaks. I thought for a moment that breqn is just a scam and only changes the default between "linebreak" and "no linebreak", but then I discovered that something like \begin{dmath*} a+a+...\end{dmath*} actually produces what I'd want - it tries to fit as many as in a row as possible, but if there's too much of them moves them to the next line. Could it do the same for me but with =s?

Answer 1

You can use inline formulas, that are broken into lines:

\documentclass{article}
\usepackage{amsmath,varwidth}

\newenvironment{breakmath}[1][0.8]
 {\begin{equation*}\begin{varwidth}{#1\columnwidth}\raggedright$\displaystyle}
 {$\end{varwidth}\end{equation*}}

\begin{document}

\begin{breakmath}
a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a
\end{breakmath}

\end{document}

The optional parameter specifies the maximum width allowed, default 80% of the column width. So, if you call \begin{breakmath}[0.7] you get maximum width 70%.

If you prefer equals signs at the beginning of the line, you can do as follows:

\documentclass{article}
\usepackage{amsmath,varwidth}

\newenvironment{breakmath}[1][0.8\columnwidth]
 {\begin{equation*}\begin{varwidth}{#1}\raggedright\makebreakequals$\displaystyle}
 {$\end{varwidth}\end{equation*}}

\newcommand{\breakequals}{%
  \penalty\relpenalty\stdequals\nobreak
}
\newcommand{\makebreakequals}{%
  \edef\stdequals{\mathchar\the\mathcode`=\relax}%
  \begingroup\lccode`~=`=\lowercase{\endgroup\let~}\breakequals
  \mathcode`=="8000
}

\begin{document}

\begin{breakmath}
a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a=a
\end{breakmath}

\end{document}