对于
(cond first-clause
rest-clause)
用过程 check 检查 first-clause ：
(check? first-clause)
看看是否符合以下格式：
'(value => proc)
如何是的话，将表达式 (cond first-clause rest-clause) 转换成：
(let ((value (car first-clause))
(proc (caddr first-clause)))
(if (true? value)
(proc value)
rest-clause))