(Lat. I do that you may do.) <A species of contract in the ‘Civil law (being one of the innominate contracts) which. occurs when I agree with a man to do his work for him if he will do mine for me; or if two persons agree to marry together, or to do any other positive acts on both sides; or it may be to forbear on one side in consideration of something done on the other. 2 Bl. Comm. 444