Using the Database

Recall the filter procedure:

          

     (define filter
       (lambda (ok? lst)
         (cond ((null? lst) '())
               ((ok? (car lst)) (cons (car lst)
                                      (filter ok? (cdr lst))))
               (else (filter ok? (cdr lst))))))

     (filter odd? '(1 2 3 4 5 6 7 8 9 10)) ⇒ (1 3 5 7 9)