S9 LIB  (unless <test-expression> <body>)  ==>  object
        (when <test-expression> <body>)    ==>  object

        (load-from-library "when.scm")

The WHEN syntax first evaluates <test-expression>. When it evaluates
to a true value, it also evaluates <body>, which is a sequence of
expressions. The expressions will be evaluated in order and the value
of the last expression will be returned. When <test-expression>
evaluates to #f, WHEN does not evaluate its <body> and returns an
unspecific value.

UNLESS is evaluates its body only if <test-expression> eveluates to #F.
It evaluates its <body> exactly if WHEN would not evaluate it and vice
versa.

(when (= 1 1) 'foo 'bar 'baz)    ==>  'baz
(unless (= 1 2) 'foo 'bar 'baz)  ==>  'baz
