[Tinyos-help] async/sync events preemption

[Frigging]

Hi,
I'm a little bit confused.
I'm wondering, but:
1) only async events can preempt tasks?
2) async events can preempt sync commands/events?
3) therefore, sync events can't peempt task or other sync commands/events,
is it?
4) but if a async function can preempt other async function where did handle
the policy, and where did save the program counter when there's preemption?
5) if while interrupt is handling, an other interrupt comes, will it be
dropped?

Sorry for my silly questions :-(

Thanks in advance