/// turn to face a given world-coordinate point
void faceToward(const Vec3d& p, double amt=1.) {
Vec3d rotEuler;
Vec3d target(p - pos());
target.normalize();
Quatd rot = Quatd::getRotationTo(uf(), target);
rot.toEuler(rotEuler);
mTurn.set(rotEuler * amt);
}
void turn(const Quatd& v) {
v.toEuler(mSpin1);
}