surface mandelbrot(float maxIter=64) {
  varying float zs = 0, zt = 0, ss = 0, tt = 0;

  varying float iter;
  for(iter = 0; iter < maxIter && ss + tt < 4); iter += 1) {
    ss = zs*zs;
    tt = zt*zt;
    zt = 2.0*zs*zt + t;
    zs = ss - tt + s;
  }

  Ci = color spline(iter/maxIter, /*...*/);
}