Xenops doesn't work with LIPICs class

Hi, I just started porting an article in the LIPICs style, which is available here https://submission.dagstuhl.de/series/details/5#author
and the rendering of any math in that style fails, with an error saying that it was not able to locate `\begin{document}`.

I perfectly understand that this being a specific custom document class defined for a journal, it is probably a low priority.

For the moment I am deactivating `xenops-mode` altogether for editing those files, and I am merely submitting this issue for the sake of completeness. If I have time to tinker with it a little afterwards, I might try and fix things for this class myself and submit a PR, but for now I need to write the article

The issue names no repository files or tests. Begin by reproducing the failure with a LIPICs document while xenops-mode is enabled, then trace how the custom document class is handled; done means math in that document renders without the reported inability to locate \\begin{document} error.

Written by the indexing model from the issue text.