K991 :

a^4 b^5 c^6 x^2 y-b^7 c^8 x^2 y-a^5 b^4 c^6 x y^2+a^7 c^8 x y^2-a^4 b^6 c^5 x^2 z+b^8 c^7 x^2 z+a^6 b^4 c^5 y^2 z-a^8 c^7 y^2 z-a^7 b^8 x z^2+a^5 b^6 c^4 x z^2+a^8 b^7 y z^2-a^6 b^5 c^4 y z^2=0