\input regression-test.tex
\DocumentMetadata{}
\documentclass{article}
\usepackage[mode=article]{osglecture-modes}
\ExplSyntaxOn
\cs_new_protected:Npn \test_keep_end:
  { \END \osglecture_modes_scan: }
\osglecture_modes_declare_allowed_command:nn { END } { test_keep_end: }
\ExplSyntaxOff

\NewDocumentEnvironment{task}{}{\TYPE{TASK BEGIN}}{\TYPE{TASK END}}
\DeclareLectureKeepEnvironment{task}

\DeclareLectureMode{solutions}
\NewDocumentEnvironment{solution}{}{\TYPE{SOLUTION BEGIN}}{\TYPE{SOLUTION END}}
\DeclareLectureKeepEnvironment[mode=solutions]{solution}

\title{Exam}
\author{}
\date{}

\begin{document}
\START
\mode<solutions>
\maketitle
\foobar
\begin{task}
  \TYPE{TASK CONTENT}
\end{task}
\begin{solution}
  \TYPE{SOLUTION CONTENT}
\end{solution}
\begin{unregisteredenv}
  \TYPE{SHOULD NOT APPEAR}
\end{unregisteredenv}
\END
\end{document}