Exercise 3.6.  It is useful to be able to reset a random-number 
generator to produce a sequence starting from a given value. Design a 
new rand procedure that is called with an argument that is either the 
symbol generate or the symbol reset and behaves as follows: (rand 
'generate) produces a new random number; ((rand 'reset) <new-value>) 
resets the internal state variable to the designated <new-value>. Thus, 
by resetting the state, one can generate repeatable sequences. These are 
very handy to have when testing and debugging programs that use random 
numbers.

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(define rand
  (let ((x random-init))
    (lambda (m)
      (cond ((eq? m 'reset) (lambda (y) (set! x y)))
            ((eq? m 'generate) (set! x (rand-update x)) x)
            (else (error "hm..."))))))