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      Create a persistent connection for XPA calls.
     
XPA_Type XPAOpen()
XPA_Type XPAOpen( String_Type mode ) 
      
	Explicitly create a persistent XPA_Type handle for use in
	subsequent XPA calls (analogous to the way a file pointer
	is used for I/O).  This is most useful for high-volume
	traffic, to avoid the cost of setting up and tearing down
	the communications channel during each message exchange.
	The mode parameter is currently ignored by XPA, and so may
	be omitted with no consequence.
       
chips> xp = XPAOpen()
chips> ns = XPAGet( xp, "xpans", "" )
chips> fname = XPAGet( xp, "prism", "filename" )
chips> XPAClose( xp )
	  
	    Here we create a persistent connection using XPAOpen() and
	    then use it in two calls: the first to find the list of
	    all XPA access points (from "xpans") and the second
	    to find the filename currently being displayed
	    by prism.
	    We then use XPAClose() to close the connection.
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