SGML's CONCUR feature (criticized but not described in that Wikipedia article) allows tags to have optional name groups specifying one or more document type nam

SGML's CONCUR feature (criticized but not described in that Wikipedia article) allows tags to have optional name groups specifying one or more document type names (that must be declared in the prolog) to which the tag applies, and allows tag pairs with disjoint document type name qualifiers to overlap like this: <(a)x>bla <(b|c)y>bla</(a)x></(b|c)y> Traditionally used for poetry and lyrics/drama but could also be useful for postal addresses, lyrics in certain types of musical notation, in translations, and maybe specific text apps such as subtitles/tracks for the hearing impaired. Basically, wherever there's a desire to markup text in more than a single hierarchy.