The gitlab.common-lisp.net system has migrated to a new host. One consequence of this is that the remotes for all cloned repos must now refer to the host gitlab.common-lisp.net rather than simply common-lisp.net.
You can see your current remotes from a cloned directory with git remote -v
If you see common-lisp.net in there without the gitlab. in front of it, then please update your remotes by using e.g. the following command:
git remote set-url origin git@gitlab.common-lisp.net:/{$user|$group}/{$repo}.git (assuming your remote name is origin).
gitlab-runner users: If you are running gitlab-runner on your own infrastructure, you need to update your config to point to https://gitlab.common-lisp.net, if it was previously pointing at simply https://common-lisp.net. The common-lisp.net host can no longer be used interchangeably with gitlab.common-lisp.net as may have worked previously. N.B. you can simply edit your gitlab-runner config and change the hostname; it should not be necessary to change anything with the token which you'll see along with that host url in the gitlab-runner config file.
When the lists are long enough, use a hashtable to determine membership. We hash both list1 and list2 to determine membership.
Add some tests for set-exclusive-or as well to test the hashtable version as well as the list version.