Sooo... When you trade food or production between your own cities... Does it TAKE the food and production from one city and BRING IT to the other? Or is the starting city not affected and the only effect is a bonus to the receiving city?
i.e.
Paris loses 2 production, Tours gains 2 production
OR
Paris doesn't lose anything, Tours gains 2 production
i.e.
Paris loses 2 production, Tours gains 2 production
OR
Paris doesn't lose anything, Tours gains 2 production