From: Nick Piggin <piggin@cyberone.com.au>

If a request is merged with another, it sometimes has to be repositioned on
the rbtree - you just do a delete then an add.  This is a quite uncommon
case though.

I changed the way adding works, so collisions must be handled by the caller
instead of being dumbly fixed by the add routine.  Unfortunately the
uncommon callers weren't handling it properly.