function Vector4:toYawPitchRoll ()
    --local ret = self:toYawPitch()
    --ret.roll = 0
    --return ret

    if math.abs(self.z) < 0.99 then
        local ret = self:toYawPitch()
        ret.roll = self.w / RAD
        return ret
    else
        return {
            yaw = self.w / RAD * sif(self.z < 0, 1, -1),
            pitch = sif(self.z > 0, -90, 90),
            roll = 0
        }
    end
end