SGML optional features

["David Santamauro" <david.santamauro@snet.net>]

This probably isn't the appropriate list for this question but there are so
many people on this list whom I've seen in connection with SGML (books
mostly) that I thought I might try.

Are there any SGML parsers that fully implement the optional features of
SGML? In particular, CONCUR and DATATAG interest me the most.

Thanks for any help.

David