Predicates

Scheme procedures that return boolean values are often called predicates. By convention (but not necessity), predicate names typically end in a question mark:

guile> (define (less-than-ten? x) (< x 10))
guile> (less-than-ten? 9)
$31 = #t
guile> (less-than-ten? 15)
$32 = #f
guile>