cross( u, v ):= [ u[2]*v[3] - u[3]*v[2], u[3]*v[1] - u[1]*v[3], u[1]*v[2] - u[2]*v[1] ];
a b sinθ
(Ay*Bz-Az*By)^2 + (Az*Bx-Ax*Bz)^2 + (Ax*By-Ay*Bx)^2 + (Ax*Bx + Ay*By + Az*Bz)^2; factor(%);