Scheme has an if procedure:

(if test-expression true-expression false-expression)

test-expression is an expression that returns a boolean value. If test-expression returns #t, the if procedure returns the value of true-expression, otherwise it returns the value of false-expression.

guile> (define a 3)
guile> (define b 5)
guile> (if (> a b) "a is greater than b" "a is not greater than b")
"a is not greater than b"

Extending LilyPond v2.25.16 (development-branch).