I've been trying to create an improvement that will give a city an extra essence, but can only be built in cities where the original essence tile yield was 1 or 2.  You can put a resource requirement for TileYieldEssence with a value of 1 or 2, but that allows any city with that value or greater to build the improvement.  I tried making the prereq value be the result of the calculation [[TileYieldEssence] + [TileYieldEssence] - 2] to prevent 3 or higher, but that didn't seem to work.

Could anyone tell me how to do this or point me to a thread that already answered this question?

Thanks!