isosurface { //-------------------------------------------------------------

  function { y - x*x + z*z  }

  threshold 0
  accuracy 0.0001
  max_gradient 5
  contained_by {box {<-1,-1,-1.5>, < 1, 1, 1.5>}}
  //open
  texture { pigment{ color rgb<0.5,1,0.7>}
            normal { bumps 0.5 scale 0.025}
            finish { phong 1}
	  }
  scale 1.0
  rotate <-20, 25, 0>
  translate <-0, 1.4, 0>
} // end of isosurface -----------------------------------------------------