dont' use the 'old' class any more.
When we reap divs for old slides, do it with a selector for
:not(.current) - this means it gets all transitional divs used by effects
without the effect author having to remember to mark them specially
fiddle with the wipe transition
although not entirely successfully: if this is a problem that can
be fixed by the sprinkling of reflow forcing pixie magic, I'm sprinkling
in the wrong places