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>The PXP user's guide</TH
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>Chapter 1. What is XML?</A
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>A complete example: The <I
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>1.1. Introduction</A
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>Extensible Markup Language</I
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generalizes the idea that text documents are typically structured in sections,
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sub-sections, paragraphs, and so on. The format of the document is not fixed
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(as, for example, in HTML), but can be declared by a so-called DTD (document
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type definition). The DTD describes only the rules how the document can be
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structured, but not how the document can be processed. For example, if you want
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to publish a book that uses XML markup, you will need a processor that converts
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the XML file into a printable format such as Postscript. On the one hand, the
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structure of XML documents is configurable; on the other hand, there is no
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longer a canonical interpretation of the elements of the document; for example
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one XML DTD might want that paragraphes are delimited by
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> tags, and another DTD expects <TT
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for the same purpose. As a result, for every DTD a new processor is required.</P
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>Although XML can be used to express structured text documents it is not limited
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to this kind of application. For example, XML can also be used to exchange
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structured data over a network, or to simply store structured data in
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files. Note that XML documents cannot contain arbitrary binary data because
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some characters are forbidden; for some applications you need to encode binary
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data as text (e.g. the base 64 encoding).</P
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>1.1.1. The "hello world" example</A
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>The following example shows a very simple DTD, and a corresponding document
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instance. The document is structured such that it consists of sections, and
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that sections consist of paragraphs, and that paragraphs contain plain text:</P
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CLASS="PROGRAMLISTING"
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><!ELEMENT document (section)+>
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<!ELEMENT section (paragraph)+>
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<!ELEMENT paragraph (#PCDATA)></PRE
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>The following document is an instance of this DTD:</P
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CLASS="PROGRAMLISTING"
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><?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE document SYSTEM "simple.dtd">
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<paragraph>This is a paragraph of the first section.</paragraph>
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<paragraph>This is another paragraph of the first section.</paragraph>
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<paragraph>This is the only paragraph of the second section.</paragraph>
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</document></PRE
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>As in HTML (and, of course, in grand-father SGML), the "pieces" of
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the document are delimited by element braces, i.e. such a piece begins with
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><name-of-the-type-of-the-piece></TT
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></name-of-the-type-of-the-piece></TT
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>, and the pieces are
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>. Unlike HTML and SGML, both start tags and
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end tags (i.e. the delimiters written in angle brackets) can never be left
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out. For example, HTML calls the paragraphs simply <TT
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because paragraphs never contain paragraphs, a sequence of several paragraphs
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CLASS="PROGRAMLISTING"
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><p>First paragraph
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<p>Second paragraph</PRE
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This is not possible in XML; continuing our example above we must always write
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CLASS="PROGRAMLISTING"
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><paragraph>First paragraph</paragraph>
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<paragraph>Second paragraph</paragraph></PRE
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The rationale behind that is to (1) simplify the development of XML parsers
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(you need not convert the DTD into a deterministic finite automaton which is
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required to detect omitted tags), and to (2) make it possible to parse the
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document independent of whether the DTD is known or not.</P
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>The first line of our sample document,
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CLASS="PROGRAMLISTING"
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><?xml version="1.0" encoding="ISO-8859-1"?></PRE
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>. It expresses that the
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document follows the conventions of XML version 1.0, and that the document is
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encoded using characters from the ISO-8859-1 character set (often known as
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"Latin 1", mostly used in Western Europe). Although the XML declaration is not
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mandatory, it is good style to include it; everybody sees at the first glance
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that the document uses XML markup and not the similar-looking HTML and SGML
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markup languages. If you omit the XML declaration, the parser will assume
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that the document is encoded as UTF-8 or UTF-16 (there is a rule that makes
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it possible to distinguish between UTF-8 and UTF-16 automatically); these
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are encodings of Unicode's universal character set. (Note that <SPAN
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predecessor "Markup", fully supports Unicode.)</P
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CLASS="PROGRAMLISTING"
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><!DOCTYPE document SYSTEM "simple.dtd"></PRE
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names the DTD that is going to be used for the rest of the document. In
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general, it is possible that the DTD consists of two parts, the so-called
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external and the internal subset. "External" means that the DTD exists as a
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second file; "internal" means that the DTD is included in the same file. In
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this example, there is only an external subset, and the system identifier
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"simple.dtd" specifies where the DTD file can be found. System identifiers are
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interpreted as URLs; for instance this would be legal:
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CLASS="PROGRAMLISTING"
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><!DOCTYPE document SYSTEM "http://host/location/simple.dtd"></PRE
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Please note that <SPAN
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> cannot interpret HTTP identifiers by default, but it is
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possible to change the interpretation of system identifiers.</P
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>The word immediately following <TT
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> determines which of
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the declared element types (here "document", "section", and "paragraph") is
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used for the outermost element, the <I
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> because the outermost element is
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><document></TT
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></document></TT
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>The DTD consists of three declarations for element types:
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>. Such a declaration has two parts:
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CLASS="PROGRAMLISTING"
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The content model is a regular expression which describes the possible inner
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structure of the element. Here, <TT
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more sections, and a <TT
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> contains one or more
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paragraphs. Note that these two element types are not allowed to contain
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arbitrary text. Only the <TT
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> element type is declared
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such that parsed character data (indicated by the symbol
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>See below for a detailed discussion of content models. </P
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>1.1.2. XML parsers and processors</A
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>XML documents are human-readable, but this is not the main purpose of this
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language. XML has been designed such that documents can be read by a program
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>. The parser checks that the document
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is well-formatted, and it represents the document as objects of the programming
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language. There are two aspects when checking the document: First, the document
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must follow some basic syntactic rules, such as that tags are written in angle
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brackets, that for every start tag there must be a corresponding end tag and so
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on. A document respecting these rules is
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>. Second, the document must match the DTD in
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which case the document is <I
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>. Many parsers check only
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on well-formedness and ignore the DTD; <SPAN
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> is designed such that it can
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even validate the document.</P
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>A parser does not make a sensible application, it only reads XML
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documents. The whole application working with XML-formatted data is called an
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>. Often XML processors convert documents into
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another format, such as HTML or Postscript. Sometimes processors extract data
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of the documents and output the processed data again XML-formatted. The parser
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can help the application processing the document; for example it can provide
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means to access the document in a specific manner. <SPAN
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object-oriented access layer specially.</P
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>1.1.3. Discussion</A
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>As we have seen, there are two levels of description: On the one hand, XML can
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define rules about the format of a document (the DTD), on the other hand, XML
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expresses structured documents. There are a number of possible applications:</P
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STYLE="list-style-type: disc"
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>XML can be used to express structured texts. Unlike HTML, there is no canonical
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interpretation; one would have to write a backend for the DTD that translates
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the structured texts into a format that existing browsers, printers
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etc. understand. The advantage of a self-defined document format is that it is
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possible to design the format in a more problem-oriented way. For example, if
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the task is to extract reports from a database, one can use a DTD that reflects
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the structure of the report or the database. A possible approach would be to
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have an element type for every database table and for every column. Once the
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DTD has been designed, the report procedure can be splitted up in a part that
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selects the database rows and outputs them as an XML document according to the
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DTD, and in a part that translates the document into other formats. Of course,
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the latter part can be solved in a generic way, e.g. there may be configurable
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backends for all DTDs that follow the approach and have element types for
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tables and columns.</P
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>XML plays the role of a configurable intermediate format. The database
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extraction function can be written without having to know the details of
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typesetting; the backends can be written without having to know the details of
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>Of course, there are traditional solutions. One can define an ad hoc
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intermediate text file format. This disadvantage is that there are no names for
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the pieces of the format, and that such formats usually lack of documentation
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because of this. Another solution would be to have a binary representation,
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either as language-dependent or language-independent structure (example of the
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latter can be found in RPC implementations). The disadvantage is that it is
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harder to view such representations, one has to write pretty printers for this
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purpose. It is also more difficult to enter test data; XML is plain text that
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can be written using an arbitrary editor (Emacs has even a good XML mode,
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PSGML). All these alternatives suffer from a missing structure checker,
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i.e. the programs processing these formats usually do not check the input file
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or input object in detail; XML parsers check the syntax of the input (the
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so-called well-formedness check), and the advanced parsers like <SPAN
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verify that the structure matches the DTD (the so-called validation).</P
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STYLE="list-style-type: disc"
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>XML can be used as configurable communication language. A fundamental problem
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of every communication is that sender and receiver must follow the same
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conventions about the language. For data exchange, the question is usually
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which data records and fields are available, how they are syntactically
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composed, and which values are possible for the various fields. Similar
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questions arise for text document exchange. XML does not answer these problems
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completely, but it reduces the number of ambiguities for such conventions: The
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outlines of the syntax are specified by the DTD (but not necessarily the
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details), and XML introduces canonical names for the components of documents
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such that it is simpler to describe the rest of the syntax and the semantics
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STYLE="list-style-type: disc"
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>XML is a data storage format. Currently, every software product tends to use
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its own way to store data; commercial software often does not describe such
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formats, and it is a pain to integrate such software into a bigger project.
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XML can help to improve this situation when several applications share the same
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syntax of data files. DTDs are then neutral instances that check the format of
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data files independent of applications. </P
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>Highlights of XML</TD
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