**''WHEN''**

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

<code lisp>
(when c f1 f2 f3)        ; syntax of WHEN

(if c (progn f1 f2 f3))  ; equivalent construction with IF
</code>

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