R4RS 4.1.5  (if <test> <consequent> <alternate>)  ==>  object
            (if <test> <consequent>)              ==>  object

Syntax: <Test>, <consequent>, and <alternate> may be arbitrary
expressions.

Semantics: An IF expression is evaluated as follows: first, <test>
is evaluated. If it yields a true value (see section see section
6.1 Booleans), then <consequent> is evaluated and its value is
returned. Otherwise <alternate> is evaluated and its value is
returned. If <test> yields a false value and no <alternate> is
specified, then the result of the expression is unspecified.

(if (> 3 2) 'yes 'no)  ==>  yes
(if (> 2 3) 'yes 'no)  ==>  no
(if (> 3 2)
    (- 3 2)
    (+ 3 2))           ==>  1
