Gr.postscript("28505-lem-ellittica.ps",
  4,4)
latox=c(-4,4); latoy=c(-4,4)
par(bg="yellow")
Gr(latox,latoy,cornice=T)
t=seq(-4,4,by=0.01)
f = function (x,y)
  (x^2+y^2)^2-(15*x^2-4*x*y+3*y^2)
valori=outer(t,t,f)
Gr.livelli(t,t,valori=valori,scritte=0)
dev.off()