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.
练习 3.6：能够将随机数生成器重置为从给定值开始产生序列是很有用的。设计一个新的 rand 过程，它以符号 generate 或符号 reset 之一为参数，行为如下：(rand 'generate) 产生一个新的随机数；((rand 'reset) ⟨new-value⟩) 将内部状态变量重置为指定的 ⟨new-value⟩。这样，通过重置状态，就可以生成可重复的序列。在测试和调试使用随机数的程序时，这非常方便。