CL-USER> (ql:quickload :filtered-functions)

CL-USER> (defgeneric frob (bar)
           (:generic-function-class filtered-functions:filtered-function))
#<FILTERED-FUNCTIONS:FILTERED-FUNCTION COMMON-LISP-USER::FROB (0)>
CL-USER> (defmethod frob (bar))
; in: DEFMETHOD FROB (T)
;     (#:METHOD-FUNCTION (BAR))
; 
; caught STYLE-WARNING:
;   The variable BAR is defined but never used.
; 
; compilation unit finished
;   caught 1 STYLE-WARNING condition
#<STANDARD-METHOD COMMON-LISP-USER::FROB (T) {100CFF0CD3}>

CL-USER> (defgeneric frob2 (bar))
#<STANDARD-GENERIC-FUNCTION COMMON-LISP-USER::FROB2 (0)>
CL-USER> (defmethod frob2 (bar))
#<STANDARD-METHOD COMMON-LISP-USER::FROB2 (T) {100D109CE3}>