XPsetDisk()
   Set the Current DOS default drive
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   Function:   XPsetDisk()

               With XPsetDisk() you can change the current DOS
               drive to any other drive. this function uses 0 to
               indicate drive A:, and not 1 as many others do.

   Syntax:     XPsetDisk(<nDisk>)

   Arguments:  <nDisk> indicates which drive will be default. 0=A
               etc..

   Returns:    Nothing, check with XPgetDisk() if the desired is
               indeed the default drive.

   Usage:      XPsetDisk(2)          /* Make C: default drive */