I finally had some time to look at this myself; the following patch
fixes the problem on my system (Debian unstable; wajig 4.0.3)
$ diff -uw /tmp/__init__.py /usr/lib/python3/dist-packages/wajig/__init__.py
--- /tmp/__init__.py 2022-07-17 17:06:15.292801740 +0200
+++ /usr/lib/python3/dist-packages/wajig/__init__.py 2022-07-17
17:06:35.828934854 +0200
@@ -712,6 +712,7 @@
function = commands.purgeremoved
parser_purgeremoved = subparsers.add_parser(
"purgeremoved",
+ parents=[parser_teach],
aliases=["purge-removed"],
description=function.__doc__,
)
Cheers,
Holger
