Add IGNORABLE to WHICHEVER
(a:whichever (foo))
expands into
(LET ((#:RANDOM-NUMBER2343 (RANDOM 1)))
(FOO))
which gives
; in: A:WHICHEVER (FOO)
; (LET ((#:RANDOM-NUMBER2343 (RANDOM 1)))
; (FOO))
;
; caught STYLE-WARNING:
; The variable #:RANDOM-NUMBER2343 is defined but never used.
; in: A:WHICHEVER (FOO)
; (FOO)
The solution is to either make the variable ignorable or return the form if there is only one form present.