[plt-dev] how to break and resume sandboxed evaluation multiple times?

1) sorry for my bad english

2) I developing "collective programming environment" with multiple sandboxes 

3) I need something that cannot work inside of current sandbox. I need to jump out from sandbox, do something and come back to sandbox with some result value.
How to do it? I try exception handling but dont understand how to break and return multiple times ... 

4)I asked this in plt-scheme at list.cs.brown.edu, but there was now answer