Ability to Trade Spells in Diplomacy

Don't think I recall this being suggested before as a diplomatic option.  Both AOW and MOM had this; I imagine the devs have it in their "to-do" list, but thought I'd put it out here in case we all missed it for some reason.  There should be a value tag to all spells, and spells from a book should be tradeable with other players, provided you both have that book (terraforming, combat, etc).