Force insertion of curly brackets

Question

This may be a LyX rather than a LaTeX thing, but here goes.

I have this snippet of code which has a macro (\TE) within a macro (\PTV)

$\left(\PTV[\TE[i]]\right)$

As it should, this fails with the error 'Argument of \\TE has an extra }'. I figured out I can fix the problem by inserting a pair of curly braces around the \TE[i] part to have

\left(\PTV[{\TE[i]}]\right)

which works just fine, until I save, close and reopen the document.

I have discovered that LyX when opening removes the braces when loading into memory (according to the 'View Source' pane). I have been doing a bit of reading around the \protect command, but have not got it to work. I'm also not entirely convinced it is a protection issue, considering the document gets saved with the desired code (it's just reopening that is the issue).

Does anyone know what's happening? or how to force LyX to keep the braces when reading in the document? Or maybe a workaround?

EDIT: As requested, \TE is defined:

\newcommandx\TE[1][usedefault, addprefix=\global, 1=]{\gamma_{#1}}

And \PTV is defined:

\newcommandx\PTV[1][usedefault, addprefix=\global, 1=]{\boldsymbol{\tau}_{#1}}

EDIT2: Bug #10562 has been filed at https://www.lyx.org/trac/ticket/10562. Bug report includes a lyx file which is a minimum reproducible example of the bug.

EDIT3: I think I can now definitively say it is a bug with Lyx. Consider the following code:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xargs}

\begin{document}
\newcommandx\TE[1][usedefault, addprefix=\global, 1=]{\gamma_{#1}}
\newcommandx\PTV[1][usedefault, addprefix=\global, 1=]{\boldsymbol{\tau}_{#1}}

$(\PTV[{\TE[i]}])$
\end{document}

Compiled using pdflatex in TexMaker, this compiles just fine. Import it into Lyx, and the issue I'm talking about occurs. I also note by moving the macros resolves the compilation issue in Lyx, but Lyx no longer recognises the macro (and therefore destroys one of the features of Lyx). i.e. :

\documentclass{article}
\usepackage{amsmath}
\usepackage{xargs}

\newcommandx\TE[1][usedefault, addprefix=\global, 1=]{\gamma_{#1}}
\newcommandx\PTV[1][usedefault, addprefix=\global, 1=]{\boldsymbol{\tau}_{#1}}

\begin{document}
$(\PTV[{\TE[i]}])$
\end{document}

Still not clear on why this is happening though...

Answer 1

Inside arguments, curly braces must be properly nested, lonely braces without partner are not allowed. However, this special treatment of braces do not apply for square brackets, there are just ordinary tokens for TeX. The use as optional argument is just a convention of LaTeX.

When TeX parses the optional argument of `\PIV':

\PTV[\TE[i]]

it looks for the first available closing bracket. It does not know about \TE and uses the closing bracket of its optional argument. The optional argument of \PTV is then:

\TE[i

That breaks the syntax for \TE.

The curly braces prevents this:

\PVI[{\TE[i]}]

Because the curly braces must match inside an argument, TeX cannot take the first closing square bracket inside the curly braces, and it uses the next closing square bracket, which happens to be the intended one.

The module xparse of LaTeX3 provides a more complicate parsing of optional arguments. To some degree, it simulates the special treatment of curly braces that they must be properly nested to the square brackets. The following example shows, that the curly braces can now be omitted. The macros with names ending in a are defined in the traditional way, the macros with names ending in b are defined via xparse:

\documentclass{article}
\usepackage{amsmath}

\usepackage{xargs}

\newcommandx\TEa[1][usedefault, addprefix=\global, 1=]{%
  \gamma\ifx\\#1\\\else_{#1}\fi
}
\newcommandx\PTVa[1][usedefault, addprefix=\global, 1=]{%
  \boldsymbol{\tau}\ifx\\#1\\\else_{#1}\fi
}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\TEb}{o}
{
  \gamma
  \IfValueT{#1}{\sb{#1}}
}
\NewDocumentCommand{\PTVb}{o}
{
  \boldsymbol{\tau}
  \IfValueT{#1}{\sb{#1}}
}
\ExplSyntaxOff

\begin{document}
$(\PTVa[{\TEa[i]}])$

$(\PTVb[{\TEb[i]}])$
\end{document}

Remarks:

• I added a simple test that suppresses the subscript if it is empty.
• \sb is used instead of _, because the underscore is a normal letter with \ExplSyntaxOn. Then it can easily be used in macro names, but does not have the subscript meaning in math mode.

Answer 2

Lyx development team have confirmed this is a bug. A fix has been developed and will be included in release 2.2.3.