[plt-scheme] Problem updating variables from threads

Hello,

Consider this code:

(define (parallel-execute . thunks)
  (for-each thread thunks))

(define x 10)

(parallel-execute (lambda () (set! x (* x x)))
                  (lambda () (set! x (+ x 1))))

(display x)

---

Always prints "10", which is surprising, since I would expect either
101 or 121 (given that set! is indeed atomic. otherwise it could be
any of more values, depending on thread switch granularity).

Eli