XPshadow()
  Just another Shadow function
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   Function:   XPshadow()

               XPshadow() is just another of those shadow functions. It uses
               SaveScreen(), XPreplAttr() and RestScreen() to make the
               shadowing, and not, such as other libraries, direct writes
               to screen. The function is as fast as any other, and respects
               the Clipper guidelines on Video I/O.

   Syntax:     XPshadow(nTop,nLeft,nBottom,nRight[,nAttr]) --> NIL

   Arguments:  The screen region where the Shadow has to be placed. The
               Shadow is placed one Line under the <nBottom> region, and
               two characters right of <nRight>. XPshadow() however, checks
               for MaxRow() and MaxCol() bounds, and eventually restricts the
               area to be shadowed. <nAttr> is optionnal, it represents the
               attribute to use as Shadow, when omitted, it uses 8 (Dark Gray)
               on Color systems and 7 (Light Gray) on Monochrome systems.

   Returns:    NIL

   Usage:      XPshadow(3,3,15,56)