set terminal postscript eps enhanced color
set output "surface.eps"
set contour
set isosamples 20
set cntrparam bspline
set cntrparam order 10
set cntrparam levels 15
set cntrparam levels incremental 0, 0.25
set noclabel
splot [-1.5:1.5][-1.5:1.5](4*(1-x*x-y*y)*(1-x*x-y*y)+2*(x*x-2)*(x*x-2)+((x+y)*(x+y)-1)*((x+y)*(x+y)-1)+((x-y)*(x-y)-1)*((x-y)*(x-y)-1)-2)/6 notitle