Discretionary equation breaking at a particular symbol

Question

I have long equations that I want LaTeX to break automatically. After some extensive searching I have been unable to work out how to achieve this effect. I have breqn in my package list, however it doesn't break the equations I have in the equation environment or math mode (or caption mode). For example it doesn't break:

A[A, k_a](M(\Box_0)) | I[k_a, d_k] | R(K[K, d_k, x]) \rightarrowtriangle A[A,k_a](M(\Box_0 | T[k_t])) | I[k_a,d_k] | I[k_t, d_k] | R(K[K,d_k,x])

I think this means I want to add \mathpunct for the \rightarrowtriangle and | symbols and only allow breaking of depth one. However I can't actually make this happen. I can't work out how to use the \mathpunct command or find what the symbol numbers of \rightarrowtriangle or | are.

Edit:

A minimal working example:

\documentclass{report}
\usepackage{stmaryrd}
\usepackage{amssymb}
\usepackage{breqn}
\begin{document}
\begin{dmath}
  A[A, k_a](M(\Box_0)) | I[k_a, d_k] | R(K[K, d_k, x]) \rightarrowtriangle A[A,k_a](M(\Box_0 | T[k_t])) | I[k_a,d_k] | I[k_t, d_k] | R(K[K,d_k,x])
\end{dmath}
\end{document}

Answer 1

Define \rightarrowtriangle in such a way it's understood by breqn; then add a level of bracing and tell breqn to break only at level 0. I used \mid, perhaps you prefer \vert for the vertical bar.

\documentclass{report}
\usepackage{amssymb,amsmath}
\usepackage{breqn}
\usepackage{stmaryrd}

\makeatletter
\ExplSyntaxOn
\cs_gset:cpx{mg@stmry}{\hexnumber@\symstmry}%
\DeclareFlexSymbol{\rightarrowtriangle}{Rel}{stmry}{5F}
\makeatother
\ExplSyntaxOff

\begin{document}
\begin{dmath}[breakdepth=0]
  \left.
  A[A, k_a](M(\Box_0)) \mid I[k_a, d_k] \mid R(K[K, d_k, x])
  \right.
  \rightarrowtriangle
  \left.
  A[A,k_a](M(\Box_0 \mid T[k_t])) \mid I[k_a,d_k] \mid I[k_t, d_k] \mid R(K[K,d_k,x])
  \right.
\end{dmath}
\end{document}

Actually there should be a stmaryrd.sym file for turning the definitions of stmaryrd.sty into something good for breqn. There isn't, at the moment.

---

Such an example confirm my opinion that it's a bad idea trying to automatize line breaks in equations.

The breqn package doesn't like |; one should use \vert, \lvert, \rvert or \mid, depending on the mathematical meaning of the bar. In this case one should use \mid; well, if one does it and makes the proper definition for \rightarrowtriangle, the result is as follows:

This is clearly unacceptable. If one uses \vert, no break can be taken. This works "automatically":

\documentclass{report}
\usepackage{amssymb}
\usepackage{breqn}
\usepackage{stmaryrd}

\makeatletter
\ExplSyntaxOn
\cs_gset:cpx{mg@stmry}{\hexnumber@\symstmry}%
\DeclareFlexSymbol{\rightarrowtriangle}{Rel}{stmry}{5F}
\makeatother
\ExplSyntaxOff

% breqn doesn't like |
\newcommand{\OR}{\hiderel{\mid}}


\begin{document}
\begin{dmath}
  A[A, k_a](M(\Box_0)) \OR I[k_a, d_k] \OR R(K[K, d_k, x])
  \rightarrowtriangle
  A[A,k_a](M(\Box_0 \OR T[k_t])) \OR I[k_a,d_k] \OR I[k_t, d_k] \OR R(K[K,d_k,x])
\end{dmath}
\end{document}

and produces

I'm asking myself where's the advantage over writing

\begin{equation}
\begin{aligned}
& A[A, k_a](M(\Box_0)) \mid I[k_a, d_k] \mid R(K[K, d_k, x]) \\
& \quad {} \rightarrowtriangle
  A[A,k_a](M(\Box_0 \mid T[k_t])) \mid I[k_a,d_k] \mid I[k_t, d_k] \mid R(K[K,d_k,x])
\end{aligned}
\end{equation}

that produces essentially the same result.