An unimplemented feature of the SGML standard and a major barrier to it's wide-spread use, CONCUR allows a single document to be parsed by multiple parsers simultanously. In theory this would be useful for parsing a document for it's physical layout rather than it's logical layout. XML threw out the CONCUR feature.
StuartYeates is a fan of CONCUR.
2 pages link to CONCUR:
