name projective coordinates with a4=-3
assume a = -3
variable X
variable Y
variable Z
neutral X = 0
neutral Y = 1
neutral Z = 0
satisfying x = X/Z
satisfying y = Y/Z
toaffine x = X/Z
toaffine y = Y/Z
tosystem X = x
tosystem Y = y
tosystem Z = 1
homogweight X = 1
homogweight Y = 1
homogweight Z = 1
