We know there is an Asha'man that can heal stilling in women as if it had never happened. Nynaeve does the same for Logain. This indicates that the opposite gender has the best ability to heal Severing in the other. (Someone posted something to that extent in another thread but I haven't gone back through all the replies to find it. Sorry.)

Now, I know that Siuan losing some of her power was a major plot point, and I know that what I am about to propose will likely never happen.

Nynaeve said she could not fix Siuan's healing because you cannot heal the same wound twice. But theoretically speaking, if Siuan were severed again could she be healed back to her original strength (by an Asha'man)? Would Nynaeve's substandard healing leave a permanent "scar" so that is now her maximum potential? Can severing be healed twice, or would it be impossible to heal again at all?

Thoughts?