Hi chris, use something like #+EXCL (defun whatever ...) #-EXCL (defun whatever ...) to distinguish between acl and not acl the #+ and #- checks *features* before it evaluates a form. each lisp has its own set of features, so #+MCL evalutes if the lisp is mcl and #+ CLISP is for clisp and so on. if you look and src/cm.lisp or src/port.lisp you will find lots of examples. clm sources are filled with these as well.