**''UNLESS''**

''UNLESS'' is another simple LISP macro built on ''IF''. In LabLISP, ''UNLESS'' is implemented as a special form. Accepts any number of arguments. The first argument - the condition - is evaluated and if it is ''NIL'', all the remaining forms are evaluated and result of the last one is returned, as in ''PROGN''. If the condition is not ''NIL'', the rest is ignored and when returns ''NIL''.

<code lisp>
(unless c f1 f2 f3)            ; syntax of UNLESS

(if (not c) (progn f1 f2 f3))  ; equivalent construction with IF
</code>

See also ''IF'', ''PROGN'', ''WHEN'', ''AND'', ''OR''