Corprus worsening effect

The branch as it is right now allows corprus to worsen every 24 hours by recording how many times the spell has worsened and silently multiplying its effects' magnitude by that value (in Spells::getMagicEffects). The only difference from vanilla is that in vanilla you may undo these (fortify and drain attribute) effects using 'damage/restore attribute' before even getting cured of corprus (that apparently applies to all diseases/abilities/curses for some reason).

However, the "proper" removal of corprus is not yet implemented. Upon removal all negative effects should be removed and the positive ones kept. The problem is that when the corprus spell is removed, all of its effects will be gone. I was thinking of recording mCured in CorprusStats and when that is true, just add all positive effects from the spell in Spells::getMagicEffects. What do you think of that solution? Is there a more general way to achieve that?

What is more, in vanilla, just like before curing corprus, these kept (fortify attribute) effects may be undone if you cast a spell with the 'damage attribute' effect on yourself. I am not sure whether that was intended or not. Hrchamd's research on the wiki mentions
>On removal, the drain spells are removed normally, while another special case interdicts any removal of positive attribute effects. Instead, a fortification is converted into a restore effect of the same magnitude.

though I don't get the last sentence. Can damage be negative?