bug#6203: Frame-local variables break let-binding

[Tom Tromey]

Tom> At least for concurrency they have the same problems as frame-locals.

Stefan> Not sure what problem you're thinking of.

All I mean is that we haven't modified the C code to make
keyboard-locals interact properly with multiple threads.  The other
differences aren't super important for concurrency in that I think they
don't make a big difference in terms of implementation effort.

Tom