Index: head/en_US.ISO8859-1/books/fdp-primer/xml-primer/chapter.xml =================================================================== --- head/en_US.ISO8859-1/books/fdp-primer/xml-primer/chapter.xml (revision 52102) +++ head/en_US.ISO8859-1/books/fdp-primer/xml-primer/chapter.xml (revision 52103) @@ -1,1415 +1,1418 @@ XML Primer Most FDP documentation is written with markup languages based on XML. This chapter explains what that means, how to read and understand the documentation source, and the XML techniques used. Portions of this section were inspired by Mark Galassi's Get Going With DocBook. Overview In the original days of computers, electronic text was simple. There were a few character sets like ASCII or EBCDIC, but that was about it. Text was text, and what you saw really was what you got. No frills, no formatting, no intelligence. Inevitably, this was not enough. When text is in a machine-usable format, machines are expected to be able to use and manipulate it intelligently. Authors want to indicate that certain phrases should be emphasized, or added to a glossary, or made into hyperlinks. Filenames could be shown in a typewriter style font for viewing on screen, but as italics when printed, or any of a myriad of other options for presentation. It was once hoped that Artificial Intelligence (AI) would make this easy. The computer would read the document and automatically identify key phrases, filenames, text that the reader should type in, examples, and more. Unfortunately, real life has not happened quite like that, and computers still require assistance before they can meaningfully process text. More precisely, they need help identifying what is what. Consider this text:
To remove /tmp/foo, use &man.rm.1;. &prompt.user; rm /tmp/foo
It is easy to see which parts are filenames, which are commands to be typed in, which parts are references to manual pages, and so on. But the computer processing the document cannot. For this we need markup. Markup is commonly used to describe adding value or increasing cost. The term takes on both these meanings when applied to text. Markup is additional text included in the document, distinguished from the document's content in some way, so that programs that process the document can read the markup and use it when making decisions about the document. Editors can hide the markup from the user, so the user is not distracted by it. The extra information stored in the markup adds value to the document. Adding the markup to the document must typically be done by a person—after all, if computers could recognize the text sufficiently well to add the markup then there would be no need to add it in the first place. This increases the cost (the effort required) to create the document. The previous example is actually represented in this document like this: paraTo remove filename/tmp/foofilename, use &man.rm.1;.para screen&prompt.user; userinputrm /tmp/foouserinputscreen The markup is clearly separate from the content. Markup languages define what the markup means and how it should be interpreted. Of course, one markup language might not be enough. A markup language for technical documentation has very different requirements than a markup language that is intended for cookery recipes. This, in turn, would be very different from a markup language used to describe poetry. What is really needed is a first language used to write these other markup languages. A meta markup language. This is exactly what the eXtensible Markup Language (XML) is. Many markup languages have been written in XML, including the two most used by the FDP, XHTML and DocBook. Each language definition is more properly called a grammar, vocabulary, schema or Document Type Definition (DTD). There are various languages to specify an XML grammar, or schema. A schema is a complete specification of all the elements that are allowed to appear, the order in which they should appear, which elements are mandatory, which are optional, and so forth. This makes it possible to write an XML parser which reads in both the schema and a document which claims to conform to the schema. The parser can then confirm whether or not all the elements required by the vocabulary are in the document in the right order, and whether there are any errors in the markup. This is normally referred to as validating the document. Validation confirms that the choice of elements, their ordering, and so on, conforms to that listed in the grammar. It does not check whether appropriate markup has been used for the content. If all the filenames in a document were marked up as function names, the parser would not flag this as an error (assuming, of course, that the schema defines elements for filenames and functions, and that they are allowed to appear in the same place). Most contributions to the Documentation Project will be content marked up in either XHTML or DocBook, rather than alterations to the schemas. For this reason, this book will not touch on how to write a vocabulary. Elements, Tags, and Attributes All the vocabularies written in XML share certain characteristics. This is hardly surprising, as the philosophy behind XML will inevitably show through. One of the most obvious manifestations of this philosophy is that of content and elements. Documentation, whether it is a single web page, or a lengthy book, is considered to consist of content. This content is then divided and further subdivided into elements. The purpose of adding markup is to name and identify the boundaries of these elements for further processing. For example, consider a typical book. At the very top level, the book is itself an element. This book element obviously contains chapters, which can be considered to be elements in their own right. Each chapter will contain more elements, such as paragraphs, quotations, and footnotes. Each paragraph might contain further elements, identifying content that was direct speech, or the name of a character in the story. It may be helpful to think of this as chunking content. At the very top level is one chunk, the book. Look a little deeper, and there are more chunks, the individual chapters. These are chunked further into paragraphs, footnotes, character names, and so on. Notice how this differentiation between different elements of the content can be made without resorting to any XML terms. It really is surprisingly straightforward. This could be done with a highlighter pen and a printout of the book, using different colors to indicate different chunks of content. Of course, we do not have an electronic highlighter pen, so we need some other way of indicating which element each piece of content belongs to. In languages written in XML (XHTML, DocBook, et al) this is done by means of tags. A tag is used to identify where a particular element starts, and where the element ends. The tag is not part of the element itself. Because each grammar was normally written to mark up specific types of information, each one will recognize different elements, and will therefore have different names for the tags. For an element called element-name the start tag will normally look like element-name. The corresponding closing tag for this element is element-name. Using an Element (Start and End Tags) XHTML has an element for indicating that the content enclosed by the element is a paragraph, called p. pThis is a paragraph. It starts with the start tag for the 'p' element, and it will end with the end tag for the 'p' element.p pThis is another paragraph. But this one is much shorter.p Some elements have no content. For example, in XHTML, a horizontal line can be included in the document. For these empty elements, XML introduced a shorthand form that is completely equivalent to the two-tag version: Using an Element Without Content XHTML has an element for indicating a horizontal rule, called hr. This element does not wrap content, so it looks like this: pOne paragraph.p hrhr pThis is another paragraph. A horizontal rule separates this from the previous paragraph.p The shorthand version consists of a single tag: pOne paragraph.p hr pThis is another paragraph. A horizontal rule separates this from the previous paragraph.p As shown above, elements can contain other elements. In the book example earlier, the book element contained all the chapter elements, which in turn contained all the paragraph elements, and so on. Elements Within Elements; <tag>em</tag> pThis is a simple emparagraphem where some of the emwordsem have been ememphasizedem.p The grammar consists of rules that describe which elements can contain other elements, and exactly what they can contain. People often confuse the terms tags and elements, and use the terms as if they were interchangeable. They are not. An element is a conceptual part of your document. An element has a defined start and end. The tags mark where the element starts and ends. When this document (or anyone else knowledgeable about XML) refers to the p tag they mean the literal text consisting of the three characters <, p, and >. But the phrase the p element refers to the whole element. This distinction is very subtle. But keep it in mind. Elements can have attributes. An attribute has a name and a value, and is used for adding extra information to the element. This might be information that indicates how the content should be rendered, or might be something that uniquely identifies that occurrence of the element, or it might be something else. An element's attributes are written inside the start tag for that element, and take the form attribute-name="attribute-value". In XHTML, the p element has an attribute called align, which suggests an alignment (justification) for the paragraph to the program displaying the XHTML. The align attribute can take one of four defined values, left, center, right and justify. If the attribute is not specified then the default is left. Using an Element with an Attribute p align="left"The inclusion of the align attribute on this paragraph was superfluous, since the default is left.p p align="center"This may appear in the center.p Some attributes only take specific values, such as left or justify. Others allow any value. Single Quotes Around Attributes p align='right'I am on the right!p Attribute values in XML must be enclosed in either single or double quotes. Double quotes are traditional. Single quotes are useful when the attribute value contains double quotes. Information about attributes, elements, and tags is stored in catalog files. The Documentation Project uses standard DocBook catalogs and includes additional catalogs for &os;-specific features. Paths to the catalog files are defined in an environment variable so they can be found by the document build tools. To Do… Before running the examples in this document, install textproc/docproj from the &os; Ports Collection. This is a meta-port that downloads and installs the standard programs and supporting files needed by the Documentation Project. &man.csh.1; users must use rehash for the shell to recognize new programs after they have been installed, or log out and then log back in again. Create example.xml, and enter this text: !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" html xmlns="http://www.w3.org/1999/xhtml" head titleAn Example XHTML Filetitle head body pThis is a paragraph containing some text.p pThis paragraph contains some more text.p p align="right"This paragraph might be right-justified.p body html Try to validate this file using an XML parser. textproc/docproj includes the xmllint validating parser. Use xmllint to validate the document: &prompt.user; xmllint --valid --noout example.xml xmllint returns without displaying any output, showing that the document validated successfully. See what happens when required elements are omitted. Delete the line with the title and title tags, and re-run the validation. &prompt.user; xmllint --valid --noout example.xml example.xml:5: element head: validity error : Element head content does not follow the DTD, expecting ((script | style | meta | link | object | isindex)* , ((title , (script | style | meta | link | object | isindex)* , (base , (script | style | meta | link | object | isindex)*)?) | (base , (script | style | meta | link | object | isindex)* , title , (script | style | meta | link | object | isindex)*))), got () This shows that the validation error comes from the fifth line of the example.xml file and that the content of the head is the part which does not follow the rules of the XHTML grammar. Then xmllint shows the line where the error was found and marks the exact character position with a ^ sign. Replace the title element. The DOCTYPE Declaration The beginning of each document can specify the name of the DTD to which the document conforms. This DOCTYPE declaration is used by XML parsers to identify the DTD and ensure that the document does conform to it. A typical declaration for a document written to conform with version 1.0 of the XHTML DTD looks like this: !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" That line contains a number of different components. <! The indicator shows this is an XML declaration. DOCTYPE Shows that this is an XML declaration of the document type. html Names the first element that will appear in the document. PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" Lists the Formal Public Identifier (FPI) Formal Public Identifier for the DTD to which this document conforms. The XML parser uses this to find the correct DTD when processing this document. PUBLIC is not a part of the FPI, but indicates to the XML processor how to find the DTD referenced in the FPI. Other ways of telling the XML parser how to find the DTD are shown later. "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" A local filename or a URL to find the DTD. > Ends the declaration and returns to the document. Formal Public Identifiers (<acronym>FPI</acronym>s) Formal Public Identifier It is not necessary to know this, but it is useful background, and might help debug problems when the XML processor can not locate the DTD. FPIs must follow a specific syntax: "Owner//Keyword Description//Language" Owner The owner of the FPI. The beginning of the string identifies the owner of the FPI. For example, the FPI "ISO 8879:1986//ENTITIES Greek Symbols//EN" lists ISO 8879:1986 as being the owner for the set of entities for Greek symbols. ISO 8879:1986 is the International Organization for Standardization (ISO) number for the SGML standard, the predecessor (and a superset) of XML. Otherwise, this string will either look like -//Owner or +//Owner (notice the only difference is the leading + or -). If the string starts with - then the owner information is unregistered, with a + identifying it as registered. ISO 9070:1991 defines how registered names are generated. It might be derived from the number of an ISO publication, an ISBN code, or an organization code assigned according to ISO 6523. Additionally, a registration authority could be created in order to assign registered names. The ISO council delegated this to the American National Standards Institute (ANSI). Because the &os; Project has not been registered, the owner string is -//&os;. As seen in the example, the W3C are not a registered owner either. Keyword There are several keywords that indicate the type of information in the file. Some of the most common keywords are DTD, ELEMENT, ENTITIES, and TEXT. DTD is used only for DTD files, ELEMENT is usually used for DTD fragments that contain only entity or element declarations. TEXT is used for XML content (text and tags). Description Any description can be given for the contents of this file. This may include version numbers or any short text that is meaningful and unique for the XML system. Language An ISO two-character code that identifies the native language for the file. EN is used for English. <filename>catalog</filename> Files With the syntax above, an XML processor needs to have some way of turning the FPI into the name of the file containing the DTD. A catalog file (typically called catalog) contains lines that map FPIs to filenames. For example, if the catalog file contained the line: PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "1.0/transitional.dtd" The XML processor knows that the DTD is called transitional.dtd in the 1.0 subdirectory of the directory that held catalog. Examine the contents of /usr/local/share/xml/dtd/xhtml/catalog.xml. This is the catalog file for the XHTML DTDs that were installed as part of the textproc/docproj port. Alternatives to <acronym>FPI</acronym>s Instead of using an FPI to indicate the DTD to which the document conforms (and therefore, which file on the system contains the DTD), the filename can be explicitly specified. The syntax is slightly different: !DOCTYPE html SYSTEM "/path/to/file.dtd" The SYSTEM keyword indicates that the XML processor should locate the DTD in a system specific fashion. This typically (but not always) means the DTD will be provided as a filename. Using FPIs is preferred for reasons of portability. If the SYSTEM identifier is used, then the DTD must be provided and kept in the same location for everyone. Escaping Back to <acronym>XML</acronym> Some of the underlying XML syntax can be useful within documents. For example, comments can be included in the document, and will be ignored by the parser. Comments are entered using XML syntax. Other uses for XML syntax will be shown later. XML sections begin with a <! tag and end with a >. These sections contain instructions for the parser rather than elements of the document. Everything between these tags is XML syntax. The DOCTYPE declaration shown earlier is an example of XML syntax included in the document. Comments An XML document may contain comments. They may appear anywhere as long as they are not inside tags. They are even allowed in some locations inside the DTD (e.g., between entity declarations). XML comments start with the string <!-- and end with the string -->. Here are some examples of valid XML comments: <acronym>XML</acronym> Generic Comments <!-- This is inside the comment --> <!--This is another comment--> <!-- This is how you write multiline comments --> <p>A simple <!-- Comment inside an element's content --> paragraph.</p> XML comments may contain any strings except --: Erroneous <acronym>XML</acronym> Comment <!-- This comment--is wrong --> To Do… Add some comments to example.xml, and check that the file still validates using xmllint. Add some invalid comments to example.xml, and see the error messages that xmllint gives when it encounters an invalid comment. Entities Entities are a mechanism for assigning names to chunks of content. As an XML parser processes a document, any entities it finds are replaced by the content of the entity. This is a good way to have re-usable, easily changeable chunks of content in XML documents. It is also the only way to include one marked up file inside another using XML. There are two types of entities for two different situations: general entities and parameter entities. General Entities General entities are used to assign names to reusable chunks of text. These entities can only be used in the document. They cannot be used in an XML context. To include the text of a general entity in the document, include &entity-name; in the text. For example, consider a general entity called current.version which expands to the current version number of a product. To use it in the document, write: paraThe current version of our product is &current.version;.para When the version number changes, edit the definition of the general entity, replacing the value. Then reprocess the document. General entities can also be used to enter characters that could not otherwise be included in an XML document. For example, < and & cannot normally appear in an XML document. The XML parser sees the < symbol as the start of a tag. Likewise, when the & symbol is seen, the next text is expected to be an entity name. These symbols can be included by using two predefined general entities: &lt; and &amp;. General entities can only be defined within an XML context. Such definitions are usually done immediately after the DOCTYPE declaration. Defining General Entities <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ENTITY current.version "3.0-RELEASE"> <!ENTITY last.version "2.2.7-RELEASE"> ]> The DOCTYPE declaration has been extended by adding a square bracket at the end of the first line. The two entities are then defined over the next two lines, the square bracket is closed, and then the DOCTYPE declaration is closed. The square brackets are necessary to indicate that the DTD indicated by the DOCTYPE declaration is being extended. Parameter Entities Parameter entities, like general entities, are used to assign names to reusable chunks of text. But parameter entities can only be used within an XML context. Parameter entity definitions are similar to those for - general entities. However, parameter entries are included + general entities. However, parameter entities are included with %entity-name;. The definition also includes the % between the ENTITY keyword and the name of the entity. For a mnemonic, think Parameter entities use the Percent symbol. Defining Parameter Entities <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ -<!ENTITY % param.some "some"> -<!ENTITY % param.text "text"> -<!ENTITY % param.new "%param.some more %param.text"> - -<!-- %param.new now contains "some more text" --> +<!ENTITY % entity "<!ENTITY version '1.0'>"> +<!-- use the parameter entity --> +%entity; ]> + + At first sight, parameter entities do not look very + useful, but they make it possible to include other files into + an XML document. To Do… Add a general entity to example.xml. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ENTITY version "1.1"> ]> html xmlns="http://www.w3.org/1999/xhtml" head titleAn Example XHTML Filetitle head <!-- There may be some comments in here as well --> body pThis is a paragraph containing some text.p pThis paragraph contains some more text.p p align="right"This paragraph might be right-justified.p pThe current version of this document is: &version;p body html Validate the document using xmllint. Load example.xml into a web browser. It may have to be copied to example.html before the browser recognizes it as an XHTML document. Older browsers with simple parsers may not render this file as expected. The entity reference &version; may not be replaced by the version number, or the XML context closing ]> may not be recognized and instead shown in the output. The solution is to normalize the document with an XML normalizer. The normalizer reads valid XML and writes equally valid XML which has been transformed in some way. One way the normalizer transforms the input is by expanding all the entity references in the document, replacing the entities with the text that they represent. xmllint can be used for this. It also has an option to drop the initial DTD section so that the closing ]> does not confuse browsers: &prompt.user; xmllint --noent --dropdtd example.xml > example.html A normalized copy of the document with entities expanded is produced in example.html, ready to load into a web browser. Using Entities to Include Files Both general and parameter entities are particularly useful for including one file inside another. Using General Entities to Include Files Consider some content for an XML book organized into files, one file per chapter, called chapter1.xml, chapter2.xml, and so forth, with a book.xml that will contain these chapters. In order to use the contents of these files as the values for entities, they are declared with the SYSTEM keyword. This directs the XML parser to include the contents of the named file as the value of the entity. Using General Entities to Include Files <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ENTITY chapter.1 SYSTEM "chapter1.xml"> <!ENTITY chapter.2 SYSTEM "chapter2.xml"> <!ENTITY chapter.3 SYSTEM "chapter3.xml"> <!-- And so forth --> ]> html xmlns="http://www.w3.org/1999/xhtml" <!-- Use the entities to load in the chapters --> &chapter.1; &chapter.2; &chapter.3; html When using general entities to include other files within a document, the files being included (chapter1.xml, chapter2.xml, and so on) must not start with a DOCTYPE declaration. This is a syntax error because entities are low-level constructs and they are resolved before any parsing happens. Using Parameter Entities to Include Files Parameter entities can only be used inside an XML context. Including a file in an XML context can be used to ensure that general entities are reusable. Suppose that there are many chapters in the document, and these chapters were reused in two different books, each book organizing the chapters in a different fashion. The entities could be listed at the top of each book, but that quickly becomes cumbersome to manage. Instead, place the general entity definitions inside one file, and use a parameter entity to include that file within the document. Using Parameter Entities to Include Files Place the entity definitions in a separate file called chapters.ent and containing this text: <!ENTITY chapter.1 SYSTEM "chapter1.xml"> <!ENTITY chapter.2 SYSTEM "chapter2.xml"> <!ENTITY chapter.3 SYSTEM "chapter3.xml"> Create a parameter entity to refer to the contents of the file. Then use the parameter entity to load the file into the document, which will then make all the general entities available for use. Then use the general entities as before: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!-- Define a parameter entity to load in the chapter general entities --> <!ENTITY % chapters SYSTEM "chapters.ent"> <!-- Now use the parameter entity to load in this file --> %chapters; ]> html xmlns="http://www.w3.org/1999/xhtml" &chapter.1; &chapter.2; &chapter.3; html To Do… Use General Entities to Include Files Create three files, para1.xml, para2.xml, and para3.xml. Put content like this in each file: pThis is the first paragraph.p Edit example.xml so that it looks like this: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ENTITY version "1.1"> <!ENTITY para1 SYSTEM "para1.xml"> <!ENTITY para2 SYSTEM "para2.xml"> <!ENTITY para3 SYSTEM "para3.xml"> ]> html xmlns="http://www.w3.org/1999/xhtml" head titleAn Example XHTML Filetitle head body pThe current version of this document is: &version;p &para1; &para2; &para3; body html Produce example.html by normalizing example.xml. &prompt.user; xmllint --dropdtd --noent example.xml > example.html Load example.html into the web browser and confirm that the paran.xml files have been included in example.html. Use Parameter Entities to Include Files The previous steps must have completed before this step. Edit example.xml so that it looks like this: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ENTITY % entities SYSTEM "entities.ent"> %entities; ]> html xmlns="http://www.w3.org/1999/xhtml" head titleAn Example XHTML Filetitle head body pThe current version of this document is: &version;p &para1; &para2; &para3; body html Create a new file called entities.ent with this content: <!ENTITY version "1.1"> <!ENTITY para1 SYSTEM "para1.xml"> <!ENTITY para2 SYSTEM "para2.xml"> <!ENTITY para3 SYSTEM "para3.xml"> Produce example.html by normalizing example.xml. &prompt.user; xmllint --dropdtd --noent example.xml > example.html Load example.html into the web browser and confirm that the paran.xml files have been included in example.html. Marked Sections XML provides a mechanism to indicate that particular pieces of the document should be processed in a special way. These are called marked sections. Structure of a Marked Section <![KEYWORD[ Contents of marked section ]]> As expected of an XML construct, a marked section starts with <!. The first square bracket begins the marked section. KEYWORD describes how this marked section is to be processed by the parser. The second square bracket indicates the start of the marked section's content. The marked section is finished by closing the two square brackets, and then returning to the document context from the XML context with >. Marked Section Keywords <literal>CDATA</literal> These keywords denote the marked sections content model, and allow you to change it from the default. When an XML parser is processing a document, it keeps track of the content model. The content model describes the content the parser is expecting to see and what it will do with that content. The CDATA content model is one of the most useful. CDATA is for Character Data. When the parser is in this content model, it expects to see only characters. In this model the < and & symbols lose their special status, and will be treated as ordinary characters. When using CDATA in examples of text marked up in XML, remember that the content of CDATA is not validated. The included text must be check with other means. For example, the content could be written in another document, validated, and then pasted into the CDATA section. Using a <literal>CDATA</literal> Marked Section paraHere is an example of how to include some text that contains many literal&lt;literal and literal&amp;literal symbols. The sample text is a fragment of acronymXHTMLacronym. The surrounding text (para and programlisting) are from DocBook.para programlisting<![CDATA[pThis is a sample that shows some of the elements within acronymXHTMLacronym. Since the angle brackets are used so many times, it is simpler to say the whole example is a CDATA marked section than to use the entity names for the left and right angle brackets throughout.p ul liThis is a listitemli liThis is a second listitemli liThis is a third listitemli ul pThis is the end of the example.p]]>programlisting <literal>INCLUDE</literal> and <literal>IGNORE</literal> When the keyword is INCLUDE, then the contents of the marked section will be processed. When the keyword is IGNORE, the marked section is ignored and will not be processed. It will not appear in the output. Using <literal>INCLUDE</literal> and <literal>IGNORE</literal> in Marked Sections <![INCLUDE[ This text will be processed and included. ]]> <![IGNORE[ This text will not be processed or included. ]]> By itself, this is not too useful. Text to be removed from the document could be cut out, or wrapped in comments. It becomes more useful when controlled by parameter entities, yet this usage is limited to entity files. For example, suppose that documentation was produced in a hard-copy version and an electronic version. Some extra text is desired in the electronic version content that was not to appear in the hard-copy. Create an entity file that defines general entities to include each chapter and guard these definitions with a parameter entity that can be set to either INCLUDE or IGNORE to control whether the entity is defined. After these conditional general entity definitions, place one more definition for each general entity to set them to an empty value. This technique makes use of the fact that entity definitions cannot be overridden but the first definition always takes effect. So the inclusion of the chapter is controlled with the corresponding parameter entity. Set to INCLUDE, the first general entity definition will be read and the second one will be ignored. Set to IGNORE, the first definition will be ignored and the second one will take effect. Using a Parameter Entity to Control a Marked Section <!ENTITY % electronic.copy "INCLUDE"> <![%electronic.copy;[ <!ENTITY chap.preface SYSTEM "preface.xml"> ]]> <!ENTITY chap.preface ""> When producing the hard-copy version, change the parameter entity's definition to: <!ENTITY % electronic.copy "IGNORE"> To Do… Modify entities.ent to contain the following: <!ENTITY version "1.1"> <!ENTITY % conditional.text "IGNORE"> <![%conditional.text;[ <!ENTITY para1 SYSTEM "para1.xml"> ]]> <!ENTITY para1 ""> <!ENTITY para2 SYSTEM "para2.xml"> <!ENTITY para3 SYSTEM "para3.xml"> Normalize example.xml and notice that the conditional text is not present in the output document. Set the parameter entity guard to INCLUDE and regenerate the normalized document and the text will appear again. This method makes sense if there are more conditional chunks depending on the same condition. For example, to control generating printed or online text. Conclusion That is the conclusion of this XML primer. For reasons of space and complexity, several things have not been covered in depth (or at all). However, the previous sections cover enough XML to introduce the organization of the FDP documentation.