Simple use of output-translations

I want to do a simple output translation and I just can't figure out what the sexp dsl would be for that.

What I want to do. Let's say my asd file is in src/foo/proj.asd. What I want is when I build the project, the fasls go in output/foo. I just can't figure out how to set up the output translations to make this happen. I'm too stupid to grok the section in the manual about the sexp dsl for translations.

Help would be appreciated. More examples in the manual (and an updated section?) would be really helpful too.