Gr.postscript("28505-lem-iperbolica.ps",
  4,2.5)
latox=c(-1.5,1.5); latoy=c(-1,1)
par(bg="yellow")
Gr(latox,latoy,cornice=T)
t=seq(-1.5,1.5,by=0.01)
f = function (x,y) (x^2+y^2)^2-(x^2-y^2)
valori=outer(t,t,f)
Gr.livelli(t,t,valori=valori,scritte=0)
dev.off()