The 'unless' special form executes the 'expr' forms if 'test' evaluates
to a NIL value. If 'test' is
(unless NIL)                    ; returns NIL
(unless T)                      ; returns NIL
(unless NIL (print "hi") 'foo)  ; prints "hi"  returns FOO
(unless (listp "a")
        (print "not a list"))   ; prints  "not a list"
                                ; returns "not a list"
See the
unless
special form in the