It is a two-lines change that allows a defsystem form depend on a package that provides additional functionality, such as new system classes, new system options, new methods, etc.
:ASDF-DEPENDENCIES ({system-name}*)
fee.asd: (defsystem :fee :components ((:file "fee")))
foo.asd: (defsystem :foo :tests ((:system :faa)) :asdf-dependencies (:fee) :components ((:file "foo")))
(load "asdf")
;;; Loading #P"/Users/jjgarcia/devel/asdf/asdf.fas" ;;; Loading #P"/Users/jjgarcia/lib/ecl-10.3.1/CMP.fas" ;;; Loading #P"/Users/jjgarcia/lib/ecl-10.3.1/sysfun.lsp" #P"/Users/jjgarcia/devel/asdf/asdf.fas"
(asdf:load-system :foo)
; loading system definition from /Users/jjgarcia/devel/asdf/foo.asd into ; #<ASDF0 package> ;;; Loading "/Users/jjgarcia/devel/asdf/foo.asd" ; loading system definition from /Users/jjgarcia/devel/asdf/fee.asd into ; #<ASDF1 package> ;;; Loading "/Users/jjgarcia/devel/asdf/fee.asd" ; registering #<SYSTEM :FEE 11882808> as FEE ;;; Loading "/Users/jjgarcia/.cache/common-lisp/ecl-10.3.1-darwin-x86/Users/jjgarcia/devel/asdf/fee.fas" ;;; ;;; Loaded prerequisite FEE ;;; ; registering #<SYSTEM :FOO 12258736> as FOO ;;; Loading "/Users/jjgarcia/.cache/common-lisp/ecl-10.3.1-darwin-x86/Users/jjgarcia/devel/asdf/foo.fas" #<ASDF:LOAD-OP NIL 9826760>