K768 :a^2*b*x^2*y - b^2*c*x^2*y - a*b^2*x*y^2 + a^2*c*x*y^2 - a^2*c*x^2*z + b*c^2*x^2*z + b^2*c*y^2*z - a*c^2*y^2*z - a^2*b*x*z^2 + a*c^2*x*z^2 + a*b^2*y*z^2 -  b*c^2*y*z^2 = 0