# Unbalanced spacing with braces in mtpro2

The code $\LEFTRIGHT\{\}{S_n}$ produces

whereras the code $\left\{ S_n \right\}$ produces

.

Is this a bug or feature? As the content in the braces grow larger, the visual effect of the unbalance is reduced. The above output of \LEFTRIGHT really looks ugly to my eyes.

Note: my preamble has \usepackage[lite,subscriptcorrection,nofontinfo]{mtpro2}

• Note also that \PARENS{S_N} and \LEFTRIGHT(){S_n} behave differently. – User Jun 16 '16 at 6:21
• $\left\{S_n\right\}$ is wrong to begin with; so is \LEFTRIGHT, independently from the strange behavior. Use automatic sizing only when really necessary, which is not the case here. – egreg Jun 16 '16 at 7:25

The answer is in the definition of \LEFTRIGHT, which means it is so “by design”:

\def\LEFTRIGHT#1#2#3{%
\specdelim@#1%
\ifspecdelim@
\LEFTRIGHT@#1.{\vc@nt@r{#3}}%
\else
\left#1
\vc@nt@r{#3}%
\right.%
\fi
\kern-2\nulldelimiterspace\mskip-\thinmuskip
\specdelim@#2%
\ifspecdelim@
\LEFTRIGHT@.#2{\vphantom{\vc@nt@r{#3}}}%
\else
\left.%
\vphantom{\vc@nt@r{#3}}%
\right#2%
\fi}


For strange reasons, \mskip-\thinmuskip is added (it's the same as \!).

Indeed, if you try

\documentclass{article}
\usepackage[lite,subscriptcorrection,nofontinfo]{mtpro2}

\begin{document}

$\{S\}$

$\left\{S\right\}$

$\LEFTRIGHT\{\}{S\,}$

\end{document}


you get