;;; Procedure:
;;;   filter
;;; Parameters:
;;;   lst, a list
;;;   ok?, a unary predicate
;;; Purpose:
;;;   Extract all elements of the list that meet the predicate.
;;; Produces:
;;;   ok, a list of values that meet the predicates
;;; Preconditions:
;;;   The predicate can be applied to every element in the list.
;;; Postconditions:
;;;   ok contains only elements from lst
;;;   ok? holds for every element of ok
;;;   Does not modify lst
(define filter
  (lambda (lst ok?)
    (cond ((null? lst) null)
          ((ok? (car lst)) (cons (car lst) (filter (cdr lst) ok?)))
          (else (filter (cdr lst) ok?)))))