Xenops forces an article class on the preview document

When writing an document with a custom class, the xenops preview document is created with the article class. Consequently, the custom commands of the class aren't recognized and the preview document doesn't compile (even though the original one does). Here are the two files:

`
\documentclass[12pt]{fphw}

% Template-specific packages
\usepackage[utf8]{inputenc} % Required for inputting international characters
\usepackage[T1]{fontenc} % Output font encoding for international characters
\usepackage{mathpazo} % Use the Palatino font
\usepackage{amsmath}
\usepackage{graphicx} % Required for including images

\usepackage{booktabs} % Required for better horizontal rules in tables

\usepackage{listings} % Required for insertion of code
\usepackage{physics}
\usepackage{enumerate} % To modify the enumerate environment

%----------------------------------------------------------------------------------------
% ASSIGNMENT INFORMATION
%----------------------------------------------------------------------------------------

\title{Test \#1} % Assignment title

\author{Sasa Salmen} % Student name

\date{april 13th, 2022} % Due date

\institute{University of São Paulo \\ Molecular Sciences Course} % Institute or school name

\class{Mathematics IV} % Course or class name

\professor{Sonia Garcia} % Professor or teacher in charge of the assignment

%----------------------------------------------------------------------------------------

\begin{document}

\maketitle % Output the assignment title, created automatically using the information in the custom commands above

%----------------------------------------------------------------------------------------
% ASSIGNMENT CONTENT
%----------------------------------------------------------------------------------------

\section*{Question 9}
%------------------------------------------------

\subsection*{Answer}
\begin{align}
DH(x,y) =
\end{align}

% ----------------------------------------------------------------------------------
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End:
`

And the preview file:

`\documentclass{article}
\usepackage[usenames]{color}
% Template-specific packages
\usepackage[utf8]{inputenc} % Required for inputting international characters
\usepackage[T1]{fontenc} % Output font encoding for international characters
\usepackage{mathpazo} % Use the Palatino font
\usepackage{amsmath}
\usepackage{graphicx} % Required for including images
\usepackage{booktabs} % Required for better horizontal rules in tables
\usepackage{listings} % Required for insertion of code
\usepackage{physics}
\usepackage{enumerate} % To modify the enumerate environment
%----------------------------------------------------------------------------------------
% ASSIGNMENT INFORMATION
%----------------------------------------------------------------------------------------
\title{Test \#1} % Assignment title
\author{Sasa Salmen} % Student name
\date{april 13th, 2022} % Due date
\institute{University of São Paulo \\ Molecular Sciences Course} % Institute or school name
\class{Mathematics IV} % Course or class name
\professor{Sonia Garcia} % Professor or teacher in charge of the assignment
%----------------------------------------------------------------------------------------
\pagestyle{empty} % do not remove
% The settings below are copied from fullpage.sty
\setlength{\textwidth}{\paperwidth}
\addtolength{\textwidth}{-3cm}
\setlength{\oddsidemargin}{1.5cm}
\addtolength{\oddsidemargin}{-2.54cm}
\setlength{\evensidemargin}{\oddsidemargin}
\setlength{\textheight}{\paperheight}
\addtolength{\textheight}{-\headheight}
\addtolength{\textheight}{-\headsep}
\addtolength{\textheight}{-\footskip}
\addtolength{\textheight}{-3cm}
\setlength{\topmargin}{1.5cm}
\addtolength{\topmargin}{-2.54cm}

\begin{document}
\definecolor{fg}{rgb}{0.796078,0.8,0.776471}
\definecolor{bg}{rgb}{0.121569,0.141176,0.188235}

\pagecolor{bg}

{\color{fg}
\begin{align}
DH(x,y) =
\end{align}
}

\end{document}
`

The error log is:

`This is pdfTeX, Version 3.141592653-2.6-1.40.23 (TeX Live 2021/Arch Linux) (preloaded format=latex)
\write18 enabled.
entering extended mode
(/tmp/2d3a33af78b003d67e101a4ae56b5168071b6464.tex
LaTeX2e <2021-11-15> patch level 1
L3 programming layer <2022-02-24>
(/usr/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2021/10/04 v1.4n Standard LaTeX document class
(/usr/share/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texmf-dist/tex/latex/graphics/color.sty
(/usr/share/texmf-dist/tex/latex/graphics-cfg/color.cfg)
(/usr/share/texmf-dist/tex/latex/graphics-def/dvips.def)
(/usr/share/texmf-dist/tex/latex/graphics/dvipsnam.def))
(/usr/share/texmf-dist/tex/latex/base/inputenc.sty)
(/usr/share/texmf-dist/tex/latex/base/fontenc.sty)
(/usr/share/texmf-dist/tex/latex/psnfss/mathpazo.sty)
(/usr/share/texmf-dist/tex/latex/amsmath/amsmath.sty
For additional information on amsmath, use the `?' option.
(/usr/share/texmf-dist/tex/latex/amsmath/amstext.sty
(/usr/share/texmf-dist/tex/latex/amsmath/amsgen.sty))
(/usr/share/texmf-dist/tex/latex/amsmath/amsbsy.sty)
(/usr/share/texmf-dist/tex/latex/amsmath/amsopn.sty))
(/usr/share/texmf-dist/tex/latex/graphics/graphicx.sty
(/usr/share/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/share/texmf-dist/tex/latex/graphics/graphics.sty
(/usr/share/texmf-dist/tex/latex/graphics/trig.sty)
(/usr/share/texmf-dist/tex/latex/graphics-cfg/graphics.cfg)))
(/usr/share/texmf-dist/tex/latex/booktabs/booktabs.sty)
(/usr/share/texmf-dist/tex/latex/listings/listings.sty
(/usr/share/texmf-dist/tex/latex/listings/lstmisc.sty)
(/usr/share/texmf-dist/tex/latex/listings/listings.cfg))
(/usr/share/texmf-dist/tex/latex/physics/physics.sty
(/usr/share/texmf-dist/tex/latex/l3packages/xparse/xparse.sty
(/usr/share/texmf-dist/tex/latex/l3kernel/expl3.sty
(/usr/share/texmf-dist/tex/latex/l3backend/l3backend-dvips.def))))
(/usr/share/texmf-dist/tex/latex/tools/enumerate.sty)
! Undefined control sequence.
l.19 \institute
{University of São Paulo \\ Molecular Sciences Course} % Ins...

! LaTeX Error: Missing \begin{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.
...

l.19 \institute{U
niversity of São Paulo \\ Molecular Sciences Course} % Ins...

! Undefined control sequence.
l.20 \class
{Mathematics IV} % Course or class name
! Undefined control sequence.
l.21 \professor
{Sonia Garcia} % Professor or teacher in charge of the assign...

(/usr/share/texmf-dist/tex/latex/psnfss/t1ppl.fd)
(/tmp//2d3a33af78b003d67e101a4ae56b5168071b6464.aux)
(/usr/share/texmf-dist/tex/latex/psnfss/ot1ppl.fd)
(/usr/share/texmf-dist/tex/latex/psnfss/omlzplm.fd)
(/usr/share/texmf-dist/tex/latex/psnfss/omszplm.fd)
(/usr/share/texmf-dist/tex/latex/psnfss/omxzplm.fd)
(/usr/share/texmf-dist/tex/latex/psnfss/ot1zplm.fd) [1]
(/tmp//2d3a33af78b003d67e101a4ae56b5168071b6464.aux) )
(see the transcript file for additional information)
Output written on /tmp//2d3a33af78b003d67e101a4ae56b5168071b6464.dvi (1 page, 8
04 bytes).
Transcript written on /tmp//2d3a33af78b003d67e101a4ae56b5168071b6464.log.
`

Start by tracing how Xenops generates the preview document from the provided LaTeX source, comparing its \documentclass line with the generated preview document. Reproduce the fphw example and verify that the preview retains the custom class and compiles without the undefined \institute, \class, and \professor errors.

Written by the indexing model from the issue text.