The present invention relates to a variable focus lens. Furthermore, the monitor can continue to be turned over until the screen of the monitor faces the back of the main system, and then further till the monitor and the main system are in the back-to-back contact with each other. The internal combustion engine and the stepless transmission are controlled in such a manner that an actual operating point of the internal combustion engine moves on a predetermined operating line in a coordinate system defined by an engine rotational speed and engine torque. Once retrofitted, the firearm safety mechanism will have the same visual appearance, action, and \u201cfeel\u201d as the firearm with which they are familiar. The predetermined delay time is less than a period between two adjacent pulse signals, and a period between two adjacent pulse signals is divided into two sub-periods by the predetermined delay time.