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  Name:     setsnow() - set snow control checking on or off
  Usage:    [<logical>] = setsnow([<logical>])
  Params:   logical value, .T. to turn snow checking on, .F. to turn
            snow checking off. optional, if left off, then the current
            set snow value is returned as a logical
  Returns:  a logical value denoting the current state of snow
            checking. logical .T. if snow checking is on, .F. if
            it is off.

 ---------------------------------- Example ---------------------------------

                 if (vidtype() = 1)
                      setsnow(.T.)
                 endif

                 or:

                 tempscrn = initscreen()
                 temp = setsnow()

                 * turn snow control off when writing to virtual
                 * buffers for maximum speed.

                 setsnow(.F.)
                 setscreen(tempscrn)
                 * set virtual buffer to initialized screen

                 box(10,10,20,50,"",23)

                 setscreen()
                 * set all video buffers to page being displayed

                 setsnow(.T.)
                 * turn snow checking back on

  Note:     The value returned by setsnow() is the setting defined
            by the SET CLIPPER=S? command. All the FUNCky video
            routines use this value as well for snow checking.
            On most Compaq computers and some clone CGA adaptors
            snow checking is not necessary. You can arbitrarily
            turn snow checking on and off under program control
            with setsnow(). Note that on a CGA with snow control
            set to on, video display speed decreases dramatically,
            also certain functions like scroll(), cls(), shift()
            clear screen etc. blank the display to write out to
            the screen, while other functions like @ ... say
            and print() check for vertical retrace before they
            write to the video screen. On a CGA adaptor, the speed
            decrease is noticeable. If you have set the screen to
            a virtual screen and you have snow control set to on,
            you should turn it off for maximum speed, and also
            to ensure that the video page currently being displayed
            is not blanked if you scroll() or cls() when using CGA.
            When writing to a shadow video buffer, and not actual
            video memory, snow control is not necessary. Snow
            control is only necessary on CGA adaptors, but not
            all the time.