If I wanted to create a improvement that converts one type of resource into another (ie alchemists hut that turns 10 metal into one crystal), I can set the resource production to a negative figure to represent resource consumption (ie -10 metal, +1 crystal per turn).

However when the resource reaches zero, it continues into negative figures, eventually ending up with -100 metal, whilst the crystal still goes up. I suspect the answer to this is that is not doable yet, but is there any workaround to stop the improvement from working if you have zero or less resources?

I have a mod idea that is more or less based on this kind of resource system of turning one thing to another in a chain, so I can't really do much with it until i can get around this problem, which I suspect maybe of the "wait until we have more mod tools" variety.