This function returns the window identifier of the active window associated with the document specified by doc, or the current document if doc is omitted or 0.

If window is specified, this function sets the active window for the document to this window, and makes doc the current document. The keyboard focus remains in the same pane as the previous frame.

If no parameters are specified, current_window returns the window which has focus at the time the function is called. Specifying a parameter changes keyboard focus to the specified window for the frame containing that window. For example, you can determine which pane in a split window will get keyboard focus the next time the frame gets focus.