<ITALICS>this</> "inherits" its value from the nearest previous full start tag, which, in this example, is <ITALICS> (in other words, it closes the most recently opened item). The expression is thus equivalent to <ITALICS>this</ITALICS>. NET Another feature is the NET (Null End Tag) construction: <ITALICS/this/, which is structurally equivalent to <ITALICS>this</ITALICS>. Other features Additionally, the SHORTTAG NETENABL IMMEDNET feature allows shortening tags surrounding an empty text value, but forbids shortening full tags: <QUOTE></QUOTE> can be written as <QUOTE// wherein the first slash ( / ) stands for the NET-enabling "start-tag close" (NESTC), and the second slash stands for the NET. NOTE: XML defines NESTC with a /, and NET with an > (angled bracket)—hence the corresponding construct in XML appears as . The third feature is 'text on the same line', allowing a markup item to be ended with a line-end; especially useful for headings and such, requiring using either SHORTREF or DATATAG minimization. For example, if the DTD includes the following declarations: <!ELEMENT lines (line*)> <!ELEMENT line O - (#PCDATA)> <!ENTITY line-tagc "</line>"> <!SHORTREF one-line "&#RE;&#RS;" line-tagc> <!USEMAP one-line line> (and "&#RE;&#RS;" is a short-reference delimiter in the concrete syntax), then: <lines> first line second line </lines> is equivalent to: <lines> <line>first line</line> <line>second line</line> </lines> Formal characterization SGML has many features that defied convenient description with the popular formal automata theory and the contemporary parser technology of the 1980s and the 1990s. The standard warns in Annex H: A report on an early implementation of a parser for basic SGML, the Amsterdam SGML Parser, notes and specifies various differences. There appears to be no definitive classification of full SGML against a known class of formal grammar. Plausible classes may include tree-adjoining grammars and adaptive grammars. XML is described as being generally parsable like a two-level grammar for non-validated XML and a Conway-style pipeline of coroutines (lexer, parser, validator) for valid XML. The SGML productions in the ISO standard are reported to be LL(3) or LL(4). XML-class subsets are reported to be expressible using a W-grammar. According to one paper, and probably considered at an information set or parse tree level rather than a character or delimiter level: The SGML standard does not define SGML with formal data structures, such as parse trees; however, an SGML document is constructed of a rooted directed acyclic graph (RDAG) of physical storage units known as "entities", which is parsed into a RDAG