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(HTML Working Group                                           ) show
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(D. Connolly) show
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(INTERNET-DRAFT                                               ) show
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(MIT/W3C) show
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(draft-ietf-html-charset-harmful-00.txt                       May 2, 1995) show
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(Expires November, 1995) show
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(Character Set) show
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( Considered Harmful) show
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(Status of this Document) show
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(This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering) show
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(Task Force \(IETF\), its areas, and its working groups. Note that other groups may also distribute working) show
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(documents as Internet-Drafts. ) show
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(Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or) show
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(obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material) show
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(or to cite them other than as "work in progress." ) show
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(To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in) show
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(the Internet-Drafts Shadow Directories on ftp.is.co.za \(Africa\), nic.nordu.net \(Europe\), munnari.oz.au) show
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(\(Pacific Rim\), ds.internic.net \(US East Coast\), or ftp.isi.edu \(US West Coast\). ) show
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(Distribution of this document is unlimited. Please send comments to the HTML working group) show
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(\(HTML-WG\) of the Internet Engineering Task Force \(IETF\) at ) show
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(<html-wg@oclc.org>) show
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(;. Discussions) show
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(of the group are archived at ) show
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(http://www.acl.lanl.gov/HTML_WG/archives.html) show
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(. ) show
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(Abstract) show
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(The term ) show
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(character set) show
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( is often used to describe a ditigal representation of text. ASCII is perhaps the) show
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(most widely deployed representation of text, and in the interest of interoperability, information systems) show
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(on the Internet traditionally rely on it exclusively. ) show
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(The Multipurpose Internet Mail Extensions \(MIME\) introduces Internet Media Types, including text) show
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(representations besides ASCII. The Hypertext Markup Language \(HTML\) used in the World-Wide Web) show
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(is a proposed Internet Media Type. But HTML is also an application of Standard Generalized Markup) show
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(Language \(SGML\). ) show
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(In the MIME and SGML specifications, the discussion of characters representation is notoriously) show
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(complex, and apparently subtly inconsistent or incompatible. This document presents a collection of) show
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(terms intended to reconcile the two specifications and serve as a basis for rigorous discussion of) show
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(characters and their digital representations. ) show
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(Introduction) show
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(The term ) show
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(character set) show
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( is often used to describe a ditigal representation of text. The specification of such) show
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(a representation typically involves identifying a sufficiently expressive collection of characters, and) show
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(giving each of them a number. ) show
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(In conventional mathematics terminology then, a "character set" is not just a set of characters, but a) show
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(function) show
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( whose domain is a set of integers, and whose range is a set of characters. ) show
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(Some standards documents, including the SGML standard, make little or no use of such conventional) show
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(mathematical terms as function, domain and range. Perhaps the authors of those documents intend the) show
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(documents to be comprehensible without a prior understanding of mathematics. But the specification of) show
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(notions such as the conformance of an SGML document or SGML system are much more complex than) show
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(the basics of logic and mathematics. ) show
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(In his text on Calculus) show
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([Spivak]) show
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(, Michael Spivak writes: ) show
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(Every aspect of this book was influenced by the desire to present calculus not merely as a) show
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(prelude to but as the first real encounter with mathematics. Since the foundation of analysis) show
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(provided the arena in which modern modes of mathematical thinking developed, calculus) show
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(ought to be the place in which to expect, rather than avoid, the strengthening of insight with) show
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(logic. In addition to developing the students' intuition about the beautiful concepts of) show
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(analysis, it is surely equally important to persuade them that precision and rigor are neither) show
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(deterrents to intuition, nor ends in themselves, but the natural medium in which to formulate) show
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(and think about mathematical questions. ) show
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(This document is not intended as the first real encounter with mathematics. But neither will we make) show
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(any effort to avoid or apologize for mathematical terminology. The reader is referred to the large body) show
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(of literature on logic and set theory, including a history of writings on math and logic[SET] and Douglas) show
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(Hofstadter's fascinating book) show
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([GEB]) show
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(. ) show
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(Coded Character Sets) show
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(Using "character set" rather than something such as ) show
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(character table) show
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( or even ) show
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(character sequence) show
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( to) show
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(denote the functions that maps integers to characters is unfortunate, but it is water under the bridge, and) show
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(a lot of it by now. Rather than attempting to divert all that water at this point, we introduce the primitive) show
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(notion of character and use it to define the term ) show
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(coded character set) show
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( from [ISO10646] and other) show
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(standards: ) show
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(character ) show
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(An atom of information ) show
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(coded character set ) show
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(A function whose domain is a subset of the integers, and whose range is a set of characters. ) show
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(Note that by the term character, we do not mean a glyph, a name, a phoneme, nor a bit combination. A) show
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(character is simply an atomic unit of communication. It is typically a symbol whose various) show
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(representations are understood to mean the same thing by a community of people. ) show
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(It might seem more intuitive to map from characters to integers, rather than the way it is defined here.) show
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(But in practice there are some coded character sets that assign two different numbers to the same) show
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(character) show
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([Lee]) show
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(, and so the inverse is not a function in the general case. ) show
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(There are two other terms used in standards such as [ISO10646] that we define in relation to the first) show
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(two: ) show
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(code position ) show
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(An integer. A coded character set and a code position from its domain determine a character. ) show
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(character repertoire ) show
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(A set of characters; that is, the range of a coded character set. ) show
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(Character Encoding Schemes) show
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(The only practical means for exchanging information on the Internet is to represent it as a sequence of) show
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(octets \(bytes\). ) show
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(One way to transmit a sequence of characters is to agree on a coded character set and transmit the) show
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(character numbers of each of the characters. ) show
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(But in practice, characters are encoded using a variety of optimizations of this brute-force approach:) show
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(code switching techniques, escape sequences, etc. The encoding of a sequence of characters is not, in) show
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(general, the result of encoding each character independently and then concatenating them. But it is) show
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(sufficiently general to note that sequences of characters are encoded as a sequence of bytes. So we) show
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(define: ) show
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(octet ) show
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(an element of the set {0, 1, 2, ..., 255} ) show
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(character encoding scheme ) show
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(a function whose domain is the set of sequences of octets, and whose range is the set of sequences) show
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(of characters over some character repertoire. ) show
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(Representation of SGML Text Entities) show
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(An SGML document is made up of entities: a text entity called the document entity, and possibly some) show
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(other text entities and data entities. ) show
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(A text entity is a sequence of characters. The representation of a text entity is not specified by the SGML) show
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(standard. For the purpose of MIME-based interchange of SGML text entities, we define the following: ) show
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(text entity ) show
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(a sequence of characters ) show
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(message entity ) show
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(a pair \(T, OS\) where T is an Internet Media Type and OS is a sequence of octets. ) show
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(Note that each ) show
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(text/*) show
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( media type has an associated ) show
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(charset) show
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( parameter, which designates a) show
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(character encoding scheme. The character encoding scheme maps the body -- a sequence of octets -- to a) show
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(text entity -- a sequence of characters. Hence any message entity of type ) show
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(text/*) show
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( is equivalent to a text) show
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(entity. ) show
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(Numeric Character References) show
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(Numeric character references are a great source of confusion. The key insights are that: ) show
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3.345 3.345 c  fill 
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( Every SGML document has exactly one document character set, which is a coded character set ) show
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3.345 3.345 c  fill 
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( Numeric character references give code positions in the document character set ) show
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(Example: ISO2022 Encoding with ISO10646 Coded Character Set) show
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(Consider the following message entity: ) show
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(Date: Saturday, 29-Apr-95 03:53:33 GMT) show
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(MIME-version: 1.0) show
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(Content-Type: text/html; charset=iso-2022-jp) show
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(<TITLE>...</TITLE>) show
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(<BODY>) show
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(Here is some normal text.) show
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(Here is a 10646 numeric character reference \200.) show
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(Here is some ISO-2022-JP text: ...) show
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(</BODY>) show
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(To interpret the message entity, we notice that the ) show
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(Content-Type) show
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( is ) show
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(text/html) show
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(, so this represents) show
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(a text entity. The ) show
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(charset) show
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( parameter ) show
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(iso-2022-jp) show
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(, along with the octet sequence of the body,) show
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(determines a sequence of characters. The octets denoted above by '...' represent characters, as per) show
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(iso-2022-jp) show
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(. ) show
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(To parse the resulting text entity as per SGML, the sender and receiver must agree on an SGML) show
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(declaration, since none is present in the document entity. For this example, we assume that SGML) show
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(declaration specifies ISO10646 as the document character set. So the numeric character reference ) show
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(\200) show
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( is) show
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(resolved with respect to ISO10646. ) show
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(It may seem contradictory that the ) show
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(ISO-2022-JP) show
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( character encoding scheme is defined in terms of a) show
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(collection of coded character sets, none of which is ISO10646. But there is no contradiction. Each) show
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(character encoded by ) show
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(ISO-2022-JP) show
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( is in the repertoire of one of those coded character sets, each of) show
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(which is a subset of the repertoire of ISO10646. ) show
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(So while ) show
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(ISO-2022-JP) show
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( is not sufficient for every ISO10646 document, it is the case that ISO10646 is) show
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(a sufficient document character set for any entity encoded with ) show
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(ISO-2022-JP) show
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(. ) show
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(Example: Reducing the Repertoire of an Entity) show
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(Suppose we have an SGML document D whose document character set is the coded character set) show
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(ISO10646. We find the document entity DE in the form of sequence of octets OS in a disk file, encoded) show
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(using the Unicode-UCS-2 character encoding scheme. ) show
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(        Unicode-UCS-2\(OS\) = DE) show
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(We can reduce the character repertoire necessary to represent the document entity by replacing) show
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(characters outside the ISO-646-IRV character repertoire with numeric character references: ) show
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(        DE' = reduce\(DE, ISO10646, ISO-646-IRV\)) show
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(where) show
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(  reduce : SEQ\(char\) X Coded Character Set X Character Repertoire -> SEQ\(char\)) show
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(and) show
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(  reduce\(c . rest, CCS, R\) = if c in R, c . reduce\(rest, CCS, R\)) show
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(                                        else &#N; . reduce\(rest, CCS, R\)) show
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(                                        where CCS\(N\) = c) show
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(The resulting entity, DE' can then be endoded using US-ASCII ) show
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(        US-ASCII\(OS'\) = DE' = reduce\(DE, ISO10646, ISO-646-IRV\)) show
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(Hence, we can represent the document D as a message entity whose content type is "text/plain;) show
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(charset=US-ASCII" and whose body is OS'. ) show
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(Conclusion) show
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(It is critical to keep separate the notion of a simple table of characters and their numbers, i.e. a coded) show
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(character set, separate from the various algorithms to encoded sequences of characters, i.e. character) show
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(encoding schemes. This separation allows a representation of a text entity which is consistent with both) show
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(the MIME and SGML specifications. ) show
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(Acknowledgements) show
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(The idea for the title of this document actually came from John Klensin. The notion of character) show
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(encoding scheme was inspired by the MIME specification by Ned Freed. James Clark, Ed Levinson, and) show
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(several other members of the MIMESGML working group collaborated in discussions leading up to this) show
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(draft. Liam Quin from SoftQuad and Gavin Nicol from EBT have provided guidance on these issues in) show
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(the past. Erik Naggum has provided invaluable aid in understanding the SGML standard. ) show
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(References) show
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([MIME]) show
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( ) show
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(N. Borenstein and N. Freed. "MIME \(Multipurpose Internet Mail Extensions\) Part One:) show
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(Mechanisms for Specifying and Describing the Format of Internet Message Bodies." RFC 1521,) show
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(Bellcore, Innosoft, September 1993. ) show
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([ASCII]) show
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( ) show
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(US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange.) show
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(Standard ANSI X3.4-1986, ANSI, 1986. ) show
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([ISO-8859]) show
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( ) show
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(ISO 8859. International Standard -- Information Processing -- 8-bit Single-Byte Coded Graphic) show
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(Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2,) show
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(ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4,) show
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(ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic) show
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(alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8:) show
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(Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990. ) show
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([SGML]) show
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( ) show
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(ISO 8879. Information Processing -- Text and Office Systems -- Standard Generalized Markup) show
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(Language \(SGML\), 1986. ) show
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([Nicol]) show
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( ) show
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(The Multilingual World Wide Web) show
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(, Gavin T. Nicol, Electronic Book Technologies, Japan) show
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(gtn@ebt.com) show
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( ) show
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([Lee]) show
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( ) show
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(Private communication with Liam Quin, from SoftQuad. ) show
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([Spivak]) show
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( ) show
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(Spivak, Michael. Calculus. 2nd Ed. 1967 ISBN 0-914098-77-2 ) show
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([GEB]) show
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( ) show
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(Hofstadter, Douglas R. G\366del, Escher, Bach: An Eternal Golden Braid, 1979 ISBN) show
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(0-394-75682-7 ) show
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([SET] ) show
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("Investigations in the foundations of set theory I", in Jean van Heijenoort \(ed.\) _From Frege to) show
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(Godel: A Source Book in Mathematical Logic, 1879-1931_ \(Harvard U.P., 1967\) ) show
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