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  writetopage                                                           WNDWC
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  Function       Changes the video page on which the WNDWC routines write
                 without changing the viewed page.
  Syntax         void writetopage( char pagenum );
  Remarks        If another video page is available, this routine will switch
                 to write to that page.  It does nothing if pagenum is
                 already the currently written video page (qvideo_page) or if
                 the page is invalid.  All wndwstats are swapped to exact
                 same conditions as before.
  Return value   None.
  Screens        All video pages.
  EOS            Restored to new page.