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.