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