How would I go about replacing default:stone in mapgen(s) with my owns? What I mean is that, let's say, I have 4 different blocks, mymod:mystone1 to mymod:mystone4. From y=0 to y=-800, I want a layer of all four of mymod's stones, so, from 0 to -200, I'd have a layer of mystone1, then from -200 to -...