Follow ASDF3 guideline of a single primary system per .asd file

[Uthar]

ASDF3 recommends a single primary system per .asd file (e.g., FOO), with other, optional subsystems being named with a slash (e.g., FOO/BAR). This ensures that FOO/BAR can be loaded without first having to manually load FOO, as would be the case with e.g. FOO-BAR (ASDF would look for it in a non-existent foo-bar.asd).

See: info asdf find-system