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This should be deleted (but I don't know how to tag an article for deletion). The specification it describes is dead ... or, rather, it was stillborn. It was written purely as an academic exercise when everything-must-be-XML fever was at its height, and there were never two interoperating implementations. (I'm not even sure there was one). It was born as part of the ZiNG initiative, from a desire to recast ANSI/NISO Z39.50 semantics in XML, but that goal was achieved less inelegantly by the newer protocol SRU. XER was removed from the ZiNG page a couple of years before ZiNG as a whole was swallowed up by SRU. (I was on the Z39.50 implementors' group and the SRU Editorial Board, so I saw all this happen.)