(en)Return cross product.
(ja)クロス積を返します。
static Sp3dV3D cross(Sp3dV3D a, Sp3dV3D b) { return Sp3dV3D((a.y * b.z) - (a.z * b.y), (a.z * b.x) - (a.x * b.z), (a.x * b.y) - (a.y * b.x)); }