Allow me to chime in. I don't know if this is trivial from the list- abs or maybe comes as a special case in one of them. But here goes.
[list-alter-idx] takes tree inputs
and returns the original list but with element number idx altered.
Makes sense? See attached.
It's whoever not robust as if idx is greater then list-length, it
misbehaves. It depends on list-abs. It's inspired by RTC.