Umm... so what happens if something external has snagged the old hash
value and the split/recalculate happens?
I was assuming that the only thing they can use the old hash value which
affects the internals of the hash is for is passing back into a hv function
as a pre-computed hash. And for a re-hashed hash the hv functions are
throwing away any passed in pre-computed values.
You can envisage a scenario where external routines are taking the hash
values and sniffing around the guts of a hash without using the hv
functions?