(define (cheap-pre-check . args) 
  ; this can return #t of #f
  #t)

(define (expensive-calculation . args)
  ; this can return #f or some other object
  (list 'some 'object))


(define (entry-point)
  (if (not (cheap-pre-check 'foo 'bar))
    (list #f 'foobar)
    (let ((result (expensive-calculation 'baz 'moo)))
      (if result (list #t result) (list #f 'foobar)))))