asynchronous exceptions

[Matthew Fluet]

> (2) good
>   - handlers get to clean up when f() times out

I'm not sure why you wouldn't say that (1) also has this option.  Even if
f() runs in a different thread, doesn't the 'a option get returned to the
thread that calls timeLimit(t, f)?  Of course, if the current thread is
waiting for timeLimit to return, why does f() need to run in a separate
thread at all?  I think I'm a little confused on how one is supposed to
use timeLimit.