FFI, safe vs unsafe

[Claus Reinke]

> It is not like inserting yields needs to be done much at all since we have
> progress guarentees, so we know the program is doing something and on
> any blocking call that could potentially take a while, the library will
> yield for you.

where do we get the progress guarantees from? do we need a 
"yield-analysis"? something that will automatically insert yields
in the code after every n atomic steps, and complain if it cannot 
infer that some piece of code is atomic, but cannot insert a yield 
either? how much of the burden do you want to shift from the
implementer to the programmer?

cheers,
claus