Collatz Collector Program and M91 Program

I haven't posted in awhile so here is some from my code archive that I have written.
CollatzProblem and the
McCarthy91-Function.

(define (collatz n)
    (and (display n)) 
          (cond ((and (odd? n) (> n 1))
                 (collatz (+ 1 (* 3 n))))
                ((even? n) (collatz (/ n 2)))))

(define (optimized-collatz n)
  (begin (and (display n))
           (let (n (modulo 4 x))
 (define (f x)
    (cond
     ((= n 0) => (/ n 4)) 
     ((= n 1) => (* (/ 3 4) (+ 1 (g (- n 1))))) 
     ((= n 2) => (/ n 2)) 
     ((= n 3) => (* (/ 1 2) (+ 1 (* 3 n)))))))
(define (g x)
    (cond
     ((not (= n 0)) => n)
     ((= n 1) => (/ 1 4 (g (- n 1))))))))


(define (collatz-range n)
  (if (= n 0)
      (display "Finished")
      (and (display #\newline)
           (collatz n)
           (collatz-range (- n 1)))))
(define (m91 n)
   (begin (and (display n)) 
          (cond ((and (<= n 100) (> n 0))
                 (m91 (m91 (+ n 11))))
                ((and (> n 0) (- n 10))))))
(define (mc91 n)
  (display n)
  (cond ((< n 1) (display "Function only defined for positive values of n"))
        ((<= n 100) (mc91 (mc91 (+ n 11))))
        (else (- n 10))))