[Grace-core] Animations in Grace

[James Noble]

> And yes, I do see the problem: you are trying to use these exercises to motivate loops, whereas the event loop model avoids the need for having loops. 

or rather the loop is abstracted (extracted) away into a library

one possibility is to have loops in space, rather than time:
but it would be nicer not to have to choose.

in a sense, this shows a weakness of Grace's model - we don't have something like call/cc or shift/reset
so we can't implement co-routines or threading "natively".   This I think also means that we couldn't
easily implement the C# async features in a library.  But this is surmise: I don't really know

James