I'd like to add that the fact that you're supposed to use only one DEFSYSTEM form per ASD file wasn't very advertised, at least back when I wrote my first ASDF

I'd like to add that the fact that you're supposed to use only one DEFSYSTEM form per ASD file wasn't very advertised, at least back when I wrote my first ASDF systems (back when ASDF2 was still in the future) and since then one rarely delved again into full rereading of the manual.