XPfunlock()
   Unlock binary files
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   Function:   XPfunlock()

               XPfunlock(), named like this to avoid conflicts, unlocks a
               specified region of a binary file. The exact reverse of
               XPflock(). see XPflock() for more info.

               All XPflock() calls should also be followed, in any event, with
               calls to XPfunlock() with the same parameters.

   Syntax:     XPfunlock(h,nOffset,nLength) --> NIL

   Arguments:  <h> is the file's handle. <nOffset> and <nLength> are the
               exact same as used by XPflock(). Supplying another region will
               cause an error.

   Returns:    NIL

   Usage: