Chapter 6 – Document Type Definition (DTD)
description
Transcript of Chapter 6 – Document Type Definition (DTD)
2002 Prentice Hall, Inc. All rights reserved.
Chapter 6 – Document Type Definition (DTD)
Outline6.1 Introduction6.2 Parsers, Well-formed and Valid XML Documents6.3 Document Type Declaration6.4 Element Type Declarations 6.4.1 Sequences, Pipe Characters and Occurrence Indicators 6.4.2 EMPTY, Mixed Content and ANY6.5 Attribute Declarations 6.5.1 Attribute Defaults (#REQUIRED, #IMPLIED, #FIXED)6.6 Attribute Types: strings (CDATA), tokenized, enumerated 6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) 6.6.2 Enumerated Attribute Types6.7 Conditional Sections6.8 Whitespace Characters6.9 Case Study: Writing a DTD for the Day Planner Application
2002 Prentice Hall, Inc. All rights reserved.
6.1 Introduction
• Document Type Definitions (DTDs)– Define structure of XML document
• i.e., what elements, attributes, etc. are permitted in document
– XML document not required to have DTD• Usually recommended for document conformity
– Use Extended Backus-Naur Form (EBNF) grammar• Wikipedia Definition: The Backus–Naur form (also known as
BNF, the Backus–Naur formalism, Backus normal form, or Panini–Backus Form) is a metasyntax used to express context-free grammars: that is, a formal way to describe formal languages.
2002 Prentice Hall, Inc. All rights reserved.
Simple DTD (not in the Deitel Text)
<?xml version=”1.0” encoding=”UTF-8”?>
<!ELEMENT rootelement (firstelement, secondelement)>
<!ELEMENT firstelement (level1)>
<!ATTLIST firstelement position CDATA #REQUIRED>
<!ELEMENT level1 (#PCDATA | level2)*>
<!ATTLIST level1 children (0 | 1) #REQUIRED>
<!ATTLIST secondelement position CDATA #REQUIRED>
<!ELEMENT level2 (#PCDATA)>
<!ELEMENT secondelement (level1)>
2002 Prentice Hall, Inc. All rights reserved.
XML Document using a simple DTD
<?xml version=”1.0” encoding=”UTF-8”?><!DOCTYPE rootelement SYSTEM “verysimplexml.dtd”><rootelement>
<firstelement position=”1”><level1 children=”0”>This is level 1 of the nested
elements</level1></firstelement><secondelement position=”2”>
<level1 children=”1”><level2>This is level 2 of the nested
elements</level2></level1>
</secondelement></rootelement>
2002 Prentice Hall, Inc. All rights reserved.
6.2 Parsers, Well-formed and Valid XML Documents
• Parsers– Validating
• Able to read DTD
• Determine whether XML document conforms to DTD
– Valid document conforms to DTD
» Document is then well formed, by definition
» Documents can be well formed, but not valid
– Nonvalidating• Able to read DTD
• Cannot check document against DTD for conformity
2002 Prentice Hall, Inc. All rights reserved.
6.3 Document Type Declaration
• Document Type Declaration– Introduce DTDs into XML documents– Placed in XML document’s prolog (markup preceding the
root element)– Begins with <!DOCTYPE– Ends with >– Can point to
• External subsets– Declarations outside document– Exist in different file
» typically ending with .dtd extension• Internal subsets
– Declarations inside document– Visible only within document in which it resides
2002 Prentice Hall, Inc. All rights reserved.
6.3 Document Type DeclarationExternal
If the DTD is external to your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:
<!DOCTYPE root-element SYSTEM "filename">
– Example (from intro.xml Fig 6.1)
<!DOCTYPE myMessage SYSTEM "intro.dtd">
2002 Prentice Hall, Inc. All rights reserved.
6.3 Document Type DeclarationInternal
If the DTD is included in your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:
<!DOCTYPE root-element [element-declarations]>
– Example (from intro2.xml in Fig 6.7)
<!DOCTYPE myMessage [
<!ELEMENT myMessage ( message )>
<!ELEMENT message ( #PCDATA )>
<!ATTLIST message id CDATA #REQUIRED>
]>
2002 Prentice Hall, Inc. All rights reserved.
6.3 Document Type Declarationcombined
Example (from p. 136 of Deitel)
<!DOCTYPE myMessage SYSTEM “myDTD.dtd” [
<!ELEMENT myElement ( #PCDATA )>
]>
2002 Prentice Hall, Inc. All rights reserved.
Outline
u
Fig. 6.1 XML document declaring its associated DTD.
DOCTYPE starts document type declaration
Document type declaration is named myMessage
Keyword SYSTEM specifies external subset
intro.dtd is DTD
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.1: intro.xml -->
4 <!-- Using an external subset -->
5
6 <!DOCTYPE myMessage SYSTEM "intro.dtd">
7
8 <myMessage>
9 <message>Welcome to XML!</message>
10 </myMessage>
Document type declaration is named myMessage
Keyword SYSTEM specifies external subset
DOCTYPE starts document type declaration
intro.dtd is DTD
2002 Prentice Hall, Inc. All rights reserved.
6.4 Element Type Declarations
• Element type declarations– Declare elements in XML documents– Begin with <!ELEMENT, End with >– Syntax:
<!ELEMENT element-name category> or
<!ELEMENT element-name (content-model)>
<!ELEMENT myElement ( #PCDATA )>• myElement is generic identifier• Parentheses specify element’s content (content specification)• Keyword PCDATA
– Element must contain parsable character data» All text treated as markup
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.2 Validation with using an external DTD.
Declare element myMessage
Element myMessage contains child element message
Declare element message
Element message contains parsable character data
1 <!-- Fig. 6.2: intro.dtd -->
2 <!-- External declarations -->
3
4 <!ELEMENT myMessage ( message )>
5 <!ELEMENT message ( #PCDATA )>
Declare element myMessage
Element myMessage contains child element message
Declare element message
Element message contains parsable character data
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.2 Validation with using an external DTD.
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.2 Validation with using an external DTD (cont.)
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.3 Non-valid XML document.
Element myMessage’s structure does not adhere to that specified in intro.dtd
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.3 : intro-invalid.xml -->
4 <!-- Simple introduction to XML markup -->
5
6 <!DOCTYPE myMessage SYSTEM "intro.dtd">
7
8 <!-- Root element missing child element message -->
9 <myMessage>
10 </myMessage>
Element myMessage’s structure does not adhere to
that specified in intro.dtd
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.3 Non-valid XML document.
2002 Prentice Hall, Inc. All rights reserved.
6.4.1 Sequences, Pipe Characters and Occurrence Indicators
• Sequences– Specify order in which elements occur
– Comma (,) used as delimiter<!ELEMENT classroom ( teacher, student )>
2002 Prentice Hall, Inc. All rights reserved.
6.4.1 Sequences, Pipe Characters and Occurrence Indicators (cont.)
• Pipe characters (|)– Specify choices<!ELEMENT dessert ( iceCream | pastry )>
2002 Prentice Hall, Inc. All rights reserved.
6.4.1 Sequences, Pipe Characters and Occurrence Indicators (cont.)
• Occurrence indicators– Specify element’s frequency
– Plus sign (+) indicates one or more occurrences<!ELEMENT album ( song+ )>
– Asterisk (*) indicates optional element<!ELEMENT library ( book* )>
– Question mark (?) indicates element can occur only once<!ELEMENT seat ( person? )>
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.4 Occurrence indicators.
Occurrence Indicator Description
Plus sign ( + ) An element can appear any number of times, but must be appear at least once (i.e., the element appears one or more times).
Asterisk ( * ) An element is optional and if used, the element can appear any number of times (i.e., the element appears zero or more times).
Question mark ( ? ) An element is optional, and if used, the element can appear only once (i.e., the element appears zero or one times).
2002 Prentice Hall, Inc. All rights reserved.
6.4.2 EMPTY, Mixed Content and ANY
• Content specification types– EMPTY
• Elements do not contain character data
• Elements do not contain child elements<!ELEMENT oven EMPTY>
• Markup for oven element<oven/>
2002 Prentice Hall, Inc. All rights reserved.
6.4.2 EMPTY, Mixed Content and ANY
• Content specification types– Mixed content
• Combination of elements and PCDATA<!ELEMENT myMessage ( #PCDATA | message )*>
• Markup for myMessage <myMessage>Here is some text, some <message>other text</message>and <message>even more text</message> </myMessage>
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.5 Example of a mixed-content element.
Specify DTD as internal subset
Declare format as mixed content element
Elements bold and italic have PCDATA only for content specification
Element format adheres to structure in DTD
1 <?xml version = "1.0" standalone = "yes"?>
2
3 <!-- Fig. 6.5 : mixed.xml -->
4 <!-- Mixed content type elements -->
5
6 <!DOCTYPE format [
7 <!ELEMENT format ( #PCDATA | bold | italic )*>
8 <!ELEMENT bold ( #PCDATA )>
9 <!ELEMENT italic ( #PCDATA )>
10 ]>
11
12 <format>
13 This is a simple formatted sentence.
14 <bold>I have tried bold.</bold>
15 <italic>I have tried italic.</italic>
16 Now what?
17 </format>
Specify DTD as internal subset
Declare format as mixed content element
Elements bold and italic have PCDATA only for content specification
Element format adheres to structure in DTD
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.5 Example of a mixed-content element.
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.6 Illegal mixed-content element syntax.
2002 Prentice Hall, Inc. All rights reserved.
6.4.2 EMPTY, Mixed Content and ANY
• Content specification types– ANY
• Can contain any content– PCDATA, elements or combination
– Can also be empty elements
• Commonly used in early DTD-development stages
– Replace with specific content as DTD evolves
2002 Prentice Hall, Inc. All rights reserved.
6.5 Attribute Declarations
• Attribute declaration– Specifies element’s attribute list– Uses ATTLIST attribute list declaration
An attribute declaration has the following syntax:<!ATTLIST element-name attribute-name attribute-type
default-value>
DTD example:<!ATTLIST payment type CDATA "check">
XML example:<payment type="check" /><payment type=“cash" />
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.7 Declaring attributes.
Specify DTD as internal subset
Declare element myMessagewith child element message
Declare that attribute idcontain required CDATA
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.7: intro2.xml -->
4 <!-- Declaring attributes -->
5
6 <!DOCTYPE myMessage [
7 <!ELEMENT myMessage ( message )>
8 <!ELEMENT message ( #PCDATA )>
9 <!ATTLIST message id CDATA #REQUIRED>
10 ]>
11
12 <myMessage>
13
14 <message id = "445">
15 Welcome to XML!
16 </message>
17
18 </myMessage>
Specify DTD as internal subset
Declare element myMessagewith child element message
Declare that attribute idcontain required CDATA
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.7 Declaring attributes.
2002 Prentice Hall, Inc. All rights reserved.
6.5.1 Attribute Defaults (#REQUIRED, #IMPLIED, #FIXED)
• Attribute defaults– Specify attribute’s default value– #IMPLIED
• Use (application’s) default value if attribute value not specified
[from w3schools: Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.]
– #REQUIRED• Attribute must appear in element
• Document is not valid if attribute is missing
– #FIXED• Attribute value is constant
• Attribute value cannot differ in XML document
2002 Prentice Hall, Inc. All rights reserved.
6.6 Attribute Types
• Attribute types– Strings (CDATA)
• No constraints on attribute values
– Except for disallowing <, >, &, ’and ” characters
– Tokenized attributes• Constraints on permissible characters for attribute values
– Enumerated attributes• Most restrictive
• Take only one value listed in attribute declaration
2002 Prentice Hall, Inc. All rights reserved.
6.6 Attribute Types
Type Value Explanation
String CDATA The value is character data
Enumerated (en1|en2|…) The value must be one from an enumerated list
Tokenized ID The value is a unique id
IDREF The value is the id of another element
IDREFS The value is a list of other ids
NMTOKEN The value is a valid XML name
NMTOKENS The value is a list of valid XML names
ENTITY The value is an entity
ENTITIES The value is a list of entities
NOTATION The value is a name of a notation
xml: The value is a predefined xml value
2002 Prentice Hall, Inc. All rights reserved.
6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN)
• Tokenized attribute types– Restrict attribute values– ID
• Uniquely identifies an element
– IDREF• Points to elements with ID attribute
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.8 XML document with ID and IDREF attribute types.
Each shipping element has a unique identifier (shipID)
Attribute shippedBy points to shipping element by matching shipID attribute
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.8: IDExample.xml -->
4 <!-- Example for ID and IDREF values of attributes -->
5
6 <!DOCTYPE bookstore [
7 <!ELEMENT bookstore ( shipping+, book+ )>
8 <!ELEMENT shipping ( duration )>
9 <!ATTLIST shipping shipID ID #REQUIRED>
10 <!ELEMENT book ( #PCDATA )>
11 <!ATTLIST book shippedBy IDREF #IMPLIED>
12 <!ELEMENT duration ( #PCDATA )>
13 ]>
14
15 <bookstore>
16 <shipping shipID = "s1">
17 <duration>2 to 4 days</duration>
18 </shipping>
19
Each shipping element has a unique identifier (shipID)
Attribute shippedBy points to shipping element by
matching shipID attribute
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.8 XML document with ID and IDREF attribute types . (Part 2)
Declare book elements with attribute shippedBy
20 <shipping shipID = "s2">
21 <duration>1 day</duration>
22 </shipping>
23
24 <book shippedBy = "s2">
25 Java How to Program 3rd edition.
26 </book>
27
28 <book shippedBy = "s2">
29 C How to Program 3rd edition.
30 </book>
31
32 <book shippedBy = "s1">
33 C++ How to Program 3rd edition.
34 </book>
35 </bookstore>
Declare book elements with attribute shippedBy
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.7 XML document with ID and IDREF attribute types.
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.9 Error displayed by XML Validator when an invalid ID is
referenced.
Assign shippedBy (line 28) value “s3”
Outline
Assign shippedBy (line 28) value “s3”
2002 Prentice Hall, Inc. All rights reserved.
6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) (cont.)
• ENTITY tokenized attribute type– Indicate that attribute has entity for its value
– Entity declaration
<!ENTITY digits “0123456789”>
– Entity may be used as follows:<useAnEntity>&digits;</useAnEntity>
– Entity reference &digits; replaced by its value<useAnEntity>0123456789</useAnEntity>
2002 Prentice Hall, Inc. All rights reserved.
Outline
Declare entity city that refers to external document elements tour.html
Fig. 6.10XML document that contains an ENTITY attribute type.
Declare entity city that refers to external document elements tour.html
NDATA indicates that external-entity content is not XML
Attribute tour for element company requires ENTITY attribute type
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.10: entityExample.xml -->
4 <!-- ENTITY and ENTITY attribute types -->
5
6 <!DOCTYPE database [
7 <!NOTATION html SYSTEM "iexplorer">
8 <!ENTITY city SYSTEM "tour.html" NDATA html>
9 <!ELEMENT database ( company+ )>
10 <!ELEMENT company ( name )>
11 <!ATTLIST company tour ENTITY #REQUIRED>
12 <!ELEMENT name ( #PCDATA )>
13 ]>
14
15 <database>
16 <company tour = "city">
17 <name>Deitel & Associates, Inc.</name>
18 </company>
19 </database>
Declare entity city that refers to external document
elements tour.html
NDATA indicates that external-entity content is not XML
Attribute tour for element company requires
ENTITY attribute type
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.10 XML document that contains an ENTITY attribute type.
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.11 Error generated by XML Validator when a DTD contains a reference to an undefined entity.
Replace line 16<company tour = "city">
with<company tour = "country">
Outline
Replace line 16
2002 Prentice Hall, Inc. All rights reserved.
6.6.1 Tokenized Attribute Type (ID, IDREF, ENTITY, NMTOKEN) (cont.)
• NMTOKEN tokenized attribute type– “Name token”
– Value consists of letters, digits, periods, underscores, hyphens and colon characters (i.e. cannot contain spaces)
2002 Prentice Hall, Inc. All rights reserved.
6.6.2 Enumerated Attribute Types
• Enumerated attribute types– Declare list of possible values for attribute
<!ATTLIST person gender ( M | F ) “F”>
• Attribute gender can have either value M or F• F is default value
2002 Prentice Hall, Inc. All rights reserved.
6.7 Conditional Sections
• Conditional sections– Include declarations
• Keyword INCLUDE
– Exclude declarations• Keyword IGNORE
– Often used with entities• Parameter entities
– Preceded by percent character (%)
– Creates entities specific to DTD
– Can be used only inside DTD in which they are declared
2002 Prentice Hall, Inc. All rights reserved.
Outline
Entities accept and reject represent strings INCLUDE and IGNORE, respectively
Fig. 6.12Conditional sections in a DTD.
Entities accept and reject represent strings INCLUDE and IGNORE, respectively
Include this element message declaration
Exclude this element message declaration
1 <!-- Fig. 6.12: conditional.dtd -->
2 <!-- DTD for conditional section example -->
3
4 <!ENTITY % reject "IGNORE">
5 <!ENTITY % accept "INCLUDE">
6
7 <![ %accept; [
8 <!ELEMENT message ( approved, signature )>
9 ]]>
10
11 <![ %reject; [
12 <!ELEMENT message ( approved, reason, signature )>
13 ]]>
14
15 <!ELEMENT approved EMPTY>
16 <!ATTLIST approved flag ( true | false ) "false">
17
18 <!ELEMENT reason ( #PCDATA )>
19 <!ELEMENT signature ( #PCDATA )>
Entities accept and reject represent strings INCLUDE and IGNORE, respectively
Include this element message declaration
Exclude this element message declaration
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.13XML document that conforms to conditional.dtd.
1 <?xml version = "1.0" standalone = "no"?>
2
3 <!-- Fig. 6.13: conditional.xml -->
4 <!-- Using conditional sections -->
5
6 <!DOCTYPE message SYSTEM "conditional.dtd">
7
8 <message>
9 <approved flag = "true"/>
10 <signature>Chairman</signature>
11 </message>
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.13 XML document that conforms to conditional.dtd.
2002 Prentice Hall, Inc. All rights reserved.
Whitespace Characters
• Whitespace– Either preserved or normalized
• Depending on context in which it is used
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.14Processing whitespace in an XML document.
Attribute hasCDATA requires CDATA, which preserves whitespace
Other attributes normalize (do not preserve) whitespace
1 <?xml version = "1.0"?>
2
3 <!-- Fig. 6.14 : whitespace.xml -->
4 <!-- Demonstrating whitespace parsing -->
5
6 <!DOCTYPE whitespace [
7 <!ELEMENT whitespace ( hasCDATA,
8 hasID, hasNMTOKEN, hasEnumeration, hasMixed )>
9
10 <!ELEMENT hasCDATA EMPTY>
11 <!ATTLIST hasCDATA cdata CDATA #REQUIRED>
12
13 <!ELEMENT hasID EMPTY>
14 <!ATTLIST hasID id ID #REQUIRED>
15
16 <!ELEMENT hasNMTOKEN EMPTY>
17 <!ATTLIST hasNMTOKEN nmtoken NMTOKEN #REQUIRED>
18
19 <!ELEMENT hasEnumeration EMPTY>
20 <!ATTLIST hasEnumeration enumeration ( true | false )
21 #REQUIRED>
22
23 <!ELEMENT hasMixed ( #PCDATA | hasCDATA )*>
24 ]>
25
Attribute cdata requires CDATA, which preserves whitespace
Other attributes normalize (do not preserve) whitespace
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.14Processing whitespace in an XML document. (Part 2)
Whitespace preserved
Whitespace normalized
26 <whitespace>
27
28 <hasCDATA cdata = " simple cdata "/>
29
30 <hasID id = " i20"/>
31
32 <hasNMTOKEN nmtoken = " hello"/>
33
34 <hasEnumeration enumeration = " true"/>
35
36 <hasMixed>
37 This is text.
38 <hasCDATA cdata = " simple cdata"/>
39 This is some additional text.
40 </hasMixed>
41
42 </whitespace>
Whitespace preserved
Whitespace normalized
2002 Prentice Hall, Inc. All rights reserved.
Outline
Output from Fig. 6.14
Whitespace preserved
Whitespace normalized
>java Tree yeswhitespace.xmlURL: file:C:/Examplesps/Files/deleted/ch09/Tree/whitespace.xml[ document root ]+-[ element : whitespace ] +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasCDATA ] +-[ attribute : cdata ]" simple cdata “ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ]+-[ element : hasID ] +-[ attribute : id ] "i20“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasNMTOKEN ] +-[ attribute : nmtoken ] "hello“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasEnumeration ] +-[ attribute : enumeration ] "true“ +-[ ignorable ] +-[ ignorable ] +-[ ignorable ] +-[ element : hasMixed ] +-[ text ] ““ +-[ text ] " This is text.“ +-[ text ] “
Whitespace normalized
Whitespace preserved
2002 Prentice Hall, Inc. All rights reserved.
Outline
Output from Fig. 6.14
“ +-[ text ] " “ +-[ element : hasCDATA ] +-[ attribute : cdata ] " simple cdata“ +-[ text ] ““ +-[ text ] " This is some additional text.“ +-[ text ] ““ +-[ text ] " “ +-[ ignorable ] +-[ ignorable ][ document end ]
2002 Prentice Hall, Inc. All rights reserved.
6.9 Case Study: Writing a DTD for the Day Planner Application
• Continue case study from Chapter 5– External subset of DTD for day planner
2002 Prentice Hall, Inc. All rights reserved.
Outline
Fig. 6.15DTD for planner.xml.
Root element planner
Element year contains one or more date elements
Element year contains attribute value that has character data
Element date contains one or more note elements
Element date contains attributes month and day
Element note contains parsed character data and optional attribute time
1 <!-- Fig. 6.15: planner.dtd -->
2 <!-- DTD for day planner -->
3
4 <!ELEMENT planner ( year* )>
5
6 <!ELEMENT year ( date+ )>
7 <!ATTLIST year value CDATA #REQUIRED>
8
9 <!ELEMENT date ( note+ )>
10 <!ATTLIST date month CDATA #REQUIRED>
11 <!ATTLIST date day CDATA #REQUIRED>
12
13 <!ELEMENT note ( #PCDATA )>
14 <!ATTLIST note time CDATA #IMPLIED>
Root element planner contains any number of (optional) year elements
Element year contains one or more date elements
Element year contains attribute value that has character data
Element date contains one or more note elements
Element date contains attributes month and day, which
contain has character data
Element note contains parsed character data and optional attribute time
2002 Prentice Hall, Inc. All rights reserved.
Fig. 6.15 DTD for planner.xml.