Several Registers at the End of the Book
If there are several registers at the end of a volume, each register is listed in a separate element <div type="index">.
<back> <div n="1" type="index"> <head>[register title 1]</head> <list> <item>[keyword 1] <ref>[page number]</ref> </item> </list> </div> <div n="1" type="index"> <head>[register title 2]</head> <list> <item>[keyword 1] <ref>[page number]</ref> </item> </list> </div> </back>