a, b, c, d, p, q, 
a*d*b
a*d*c
a*d*(a-d)
-a^2*p-a*b*c+a*p^2
-a^2*q-a*b*c+a*q^2
-b*c*d-d^2*p+d*p^2
-b*c*d-d^2*q+d*q^2
