´x v ´x 3x4 v ´x 1x4 v x2x4 v ´x 3x4 v x4)(x3 v ´x 4) = x1x2 x3 v x1x3x4 v ´x ´x 3 1 x v ´x 2x3x4 v ´x 1x3x4 v x2x3x4 v x3x4 v v x1x2 ´x 2 3 4 v x1 ´x 3 ´x