struct Thread
	void *f
	int pc

typedef void (*func)(struct Thread *)

struct Object
	Thread *holding
	Thread *waiting

struct Int
	Object
	int i

enum { MAXTHREADS = 10 }
int qpush = 0
int qpop = 0
Thread *q[MAXTHREADS]

sched(Thread *t)
	if t
		q[qpush] = t
		if ++qpush == MAXTHREADS
			qpush = 0
		if qpush == qpop
			exits("too many threads")

get(Object *o, Thread *t)
	if o->holding == 0
		o->holding = t
		sched(t)
	else if o->waiting
		exits("broken")
	else
		o->waiting = t

put(Object *o, Thread *t)
	if o->holding == t && o->waiting
		o->holding = o->waiting
		o->waiting = 0
		sched(o->holding)

yield(Object *o, Thread *t)
	put(o, t)
	get(o, t)

connect(void **from, Object *obj, void **to)
	obj->holding = 0
	obj->waiting = 0
	*from = obj
	*to = obj

step()
	Thread *t
	if qpop == qpush
		exits("no threads")
	t = q[qpop]
	if ++qpop == MAXTHREADS
		qpop = 0
	(*(func)t->f)(t)

start(Thread *t)
	t->pc = 0
	sched(t)

run()
	while 1
		step()