How to get the current screen

[Moon@STONY-BROOK.SCRC.Symbolics.COM (David A. Moon)]

    Date: Wed, 21 Nov 1990 16:15 EST
    From: Donald H. Mitchell <dmitchell@SPIRIT.trc.amoco.com>

	Date: Tue, 20 Nov 1990 15:44-0500
	From: Moon@STONY-BROOK.SCRC.Symbolics.COM (David A. Moon)

	Calling tv:mouse-default-superior is a good answer.  Note that it takes a window as
	an optional argument, defaulting to the value of *TERMINAL-IO*.
	[...]
	Using tv:mouse-default-superior
	gives more robustness in the face of various unusual situations, for example if you have
	a color screen and a monochrome screen sharing the same mouse, it will use the screen
	the mouse is on even if the window you give is on the other screen; for menus you
	generally would rather have them appear next to the mouse.

    A curiosity flag raises which asks, "How does the program know which mouse to
    base this function on?"  Remember, when running X windows, there's a separate
    mouse for each console.

Note that it takes a window as an optional argument.  It uses the mouse
associated with that window, i.e. the mouse attached to the console that
owns the screen on which that window is displayed.