protected static void cross(_Cartesian t, ref _Cartesian u, ref _Cartesian v){
t.xs = u.ys * v.zs - u.zs * v.ys;
t.ys = u.zs * v.xs - u.xs * v.zs;
t.zs = u.xs * v.ys - u.ys * v.xs;
}
static protected void set(_Cartesian v, double x, double y, double z) {
v.xs = x;
v.ys = y;
v.zs = z;
}
