pandoc-3.0.1: test/command/8573.md
```
% pandoc -f latex -t plain
\newenvironment{myenv}[1]{Open#1}{Close}
\begin{myenv}{x}Hello\end{myenv}
^D
OpenxHelloClose
```
```
% pandoc -f latex -t plain
\newenvironment{myenv}[1]{Open#1}{Close}
\begin{myenv}{x}Hello\end{myenv}
^D
OpenxHelloClose
```