I know how to use multiple functions. Just put them in square brackets like this
plot3d([x + y, 2x + y], x=-3..3, y=-3..3)
The thing is - I don't know how to create it, where function will depend on x and y. Like if x < 0, it will choose first function. If x > 0, it will use second function. I want to use a functions like these:
