Hi I think I was able to make CL-UNIFICATION dependency on CL-PPCRE optional. It looks better in MK:DEFSYSTEM, but it works also with ASDF, at the price of loading the 'lib dependent' .asd file after the main one. In other words, you have to do (asdf:oos 'asdf:load-op :cl-unification) (asdf:oos 'asdf:load-op :cl-unification-lib) APPLY-SUBSTITUTION now almost works. (At least in Matthew's example). Cheers -- Marco Antoniotti