(define-filter unread (field)
  (getf (cdr field) :unread))

==>
; in: LAMBDA NIL
;     ((GETF (CDR FIELD) :UNREAD))
; 
; caught ERROR:
;   illegal function call

;     (SB-INT:NAMED-LAMBDA UNREAD (&REST TAGS &KEY FIELD &ALLOW-OTHER-KEYS)
;                        (BLOCK UNREAD ((GETF (CDR FIELD) :UNREAD))))
; ==>
;   #'(SB-INT:NAMED-LAMBDA UNREAD (&REST TAGS &KEY FIELD &ALLOW-OTHER-KEYS)
;                          (BLOCK UNREAD ((GETF (CDR FIELD) :UNREAD))))
; 
; caught STYLE-WARNING:
;   The variable FIELD is defined but never used.
; 
; caught STYLE-WARNING:
;   The variable TAGS is defined but never used.
; 
; compilation unit finished
;   caught 1 ERROR condition
;   caught 2 STYLE-WARNING conditions
STYLE-WARNING: redefining UNREAD in DEFUN