(let ((sys (ql-dist:dependency-tree "cl-vectors")))
  (maptree
   (lambda (s)
     (when (member
            (asdf:find-system
             (ql-dist:system-file-name s))
            '("gpl" "gpl2" "gpl3" "gplv2" ...)
            :test #'equalp
            :key #'asdf:system-license)
       (error "user freedom is put above creator's freedom")))
   sys))