(let* ((ver (lisp-implementation-version))
       (major (parse-integer ver :start 0 :end (position #\. ver))))
  #+lispworks (assert (>= 5 major) () "Requires Lispworks version 5 or above")
  #+clisp (assert (>= 2 major) () "Requires CLISP 2.n")
  )