http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-23.html#%_sec_3.4.1
(define (parallel-execute . procs) (map thread-wait (map (lambda (proc) (thread proc)) procs)))
我无法保证它与本书中定义的并行执行过程具有相同的语义,但它允许我解决练习.