Automatic choice of curly and straight extensible braces in MTPro2

Question

The MTProII fonts provide two sets of curly braces, the straight extensible ones and the curly desinged braces that extend up to 4in high.... and that is all very dandy, except that when using the {cases} construct from the amsmath it (automatically) sometimes chooses one type and sometimes choose another in a mix that does not look very nice:

\documentclass{report}
\usepackage{amsmath}
\usepackage{mtpro2}
\begin{document}
[
\begin{cases}
a_n=2 a_{n-1}+1, \quad \forall,n\geq 2,\
a_1=1.\
\end{cases}
]
[
\begin{cases}
a_n=a_{n-1}+a_{n-2},\quad \forall,n\geq 3,\
a_1=1,\
a_2=1,\
\end{cases}
]
\end{document}

Is this an error in the package that loads the fonts or something that could be controlled from the outside?

I have been looking at the code, but cannot quite locate the place where this is controlled.

Is it possible to handle this differently by using fontspec and avoiding the package mtpro2?

I am aware of the question/answer here, but that deal with a different problem -- not the automatic choice of cases.

Answer 1

First the output (code below):

Short answer

Use the patch mtpro2-patch.tex I developed over the years (my “shameless plug”).

\documentclass{report}
\usepackage{amsmath}
\usepackage{mtpro2}
\input{mtpro2-patch}% https://github.com/RuixiZhang42/font-pairing-guide/blob/master/mtpro2-patch.tex
\begin{document}
Every \verb|cases| uses curly brace:
[
\begin{cases}
a_n=2 a_{n-1}+1, \quad \forall,n\geq 2,\
a_1=1.\
\end{cases}
\qquad
\begin{cases}
a_n=a_{n-1}+a_{n-2},\quad \forall,n\geq 3,\
a_1=1,\
a_2=1,\
\end{cases}
]
\straightbraces
Now, every \verb|cases| uses straight brace:
[
\begin{cases}
a_n=2 a_{n-1}+1, \quad \forall,n\geq 2,\
a_1=1.\
\end{cases}
\qquad
\begin{cases}
a_n=a_{n-1}+a_{n-2},\quad \forall,n\geq 3,\
a_1=1,\
a_2=1,\
\end{cases}
]
\end{document}

Long answer

The following provides essentially a brief documentation for one small part of mtpro2-patch.tex.

\documentclass{report}
\usepackage{amsmath}
\usepackage{mtpro2}
\makeatletter
% First, we need a test to see which braces are in force.
% The mtpro2 package provides 3 sets of braces. We treat
% curly braces as one type, while morphed and straight
% braces as another type.
\expandafter\def\csname mtp2@lcbrace\endcsname
  {\delimiter"4266308 }% define our own left curly brace
\newcommand*@ifcurlybraces{%
  \expandafter\ifx\csname mtp2@lcbrace\endcsname\lbrace
    \expandafter@firstoftwo
  \else
    \expandafter@secondoftwo
  \fi
}
% Next, we redefine the cases environment.
% If curly braces are in force, we use the \LEFTRIGHT construction.
% Otherwise, we use the \left...\right construction.
\newsavebox\mtp@matrix@cases
\renewenvironment{cases}{%
  \matrix@check\cases
  \setbox\mtp@matrix@cases\hbox\bgroup$%
  \let@ifnextchar\new@ifnextchar
  \def\arraystretch{1.1}% less than 1000/\delimiterfactor, my personal preference
  \array{@{}l@{\quad}l@{}}%
}{%
  \endarray
  \m@th$\egroup
  @ifcurlybraces{%
    \LEFTRIGHT@\lbrace.{,\copy\mtp@matrix@cases}%
  }{%
    \left\lbrace\copy\mtp@matrix@cases\right.%
  }%
}
\makeatother
\begin{document}
Every \verb|cases| uses curly brace:
[
\begin{cases}
a_n=2 a_{n-1}+1, \quad \forall,n\geq 2,\
a_1=1.\
\end{cases}
\qquad
\begin{cases}
a_n=a_{n-1}+a_{n-2},\quad \forall,n\geq 3,\
a_1=1,\
a_2=1,\
\end{cases}
]
\straightbraces
Now, every \verb|cases| uses straight brace:
[
\begin{cases}
a_n=2 a_{n-1}+1, \quad \forall,n\geq 2,\
a_1=1.\
\end{cases}
\qquad
\begin{cases}
a_n=a_{n-1}+a_{n-2},\quad \forall,n\geq 3,\
a_1=1,\
a_2=1,\
\end{cases}
]
\end{document}