tree and as in a concrete syntax tree. In the SGML usage, a concrete syntax is a set of specific delimiters, while the abstract syntax is the set of names for the delimiters. The XML Infoset corresponds more to the programming language notion of abstract syntax introduced by John McCarthy. Derivatives XML The W3C XML (Extensible Markup Language) is a profile (subset) of SGML designed to ease the implementation of the parser compared to a full SGML parser, primarily for use on the World Wide Web. In addition to disabling many SGML options present in the reference syntax (such as omitting tags and nested subdocuments) XML adds a number of additional restrictions on the kinds of SGML syntax. For example, despite enabling SGML shortened tag forms, XML does not allow unclosed start or end tags. It also relied on many of the additions made by the WebSGML Annex. XML currently is more widely used than full SGML. XML has lightweight internationalization based on Unicode. Applications of XML include XHTML, XQuery, XSLT, XForms, XPointer, JSP, SVG, RSS, Atom, XML-RPC, RDF/XML, and SOAP. HTML While HTML (Hyper Text Markup Language) was developed partially independently and in parallel with SGML, its creator, Tim Berners-Lee, intended it to be an application of SGML. The design of HTML was therefore inspired by SGML tagging, but, since no clear expansion and parsing guidelines were established, most actual HTML documents are not valid SGML documents. Later, HTML was reformulated (version 2.0) to be more of an SGML application; however, the HTML markup language has many legacy- and exception-handling features that differ from SGML's requirements. HTML 4 is an SGML application that fully conforms to ISO 8879 – SGML. The charter for the 2006 revival of the World Wide Web Consortium HTML Working Group says, "the Group will not assume that an SGML parser is used for 'classic HTML'". Although HTML syntax closely resembles SGML syntax with the default reference concrete syntax, HTML5 abandons any attempt to define HTML as an SGML application, explicitly defining its own parsing rules, which more closely match existing implementations and documents. It does, however, define an alternative XHTML serialization, which conforms to XML and therefore to SGML as well. OED The second edition of the Oxford English Dictionary (OED) is entirely marked up with an SGML-based markup language using the LEXX text editor. The third edition is marked up as XML. Others