Implicit equation

using AbstractPlotting

r = range(-5, stop = 5, length = 400)
(a, b) = -1, 2
z = ((x,y) -> y.^4 - x.^4 + a .* y.^2 + b .* x.^2).(r, r')
z2 =  z .* (abs.(z) .< 250)
contour(r, r, z2)