I'd like to draw a line on a surface[x,y,z] at z=0. Furthermore, I like the surface to be one color when z < 0 and another when z > 0.
I've tried two approaches, but neither worked for me:
1. Colorfunction - I cannot specify a colorfunction as a function of z, only of x or y.
2. Constraint - I could draw a constraint on the surface. Unfortunately I do not get closed form solutions for x and y when z=0.