source("http://macosa.dima.unige.it/R/r.R")
f1 = function(x,y) x^2+4*y^2-4*x*y-y
f2 = function(x,y) x^2-y^2+x*y+x-1
f3 = function(x,y) x^2+2*y^2-x*y+x+y-1
BF=4; HF=4; PLANE(-3,2,-3,2)
CURVE(f1,"blue"); CURVE(f2,"red"); CURVE(f3,"green4")