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The use of substitution groups in a schema doesn't mean you're straying
dangerously over the syntax-semantics boundary. It just means that
the _syntax_ of a number of your elements has something in common. If
I write:
<xs:element name="para">
<xs:complexType mixed="true">
<xs:sequence>
<xs:element ref="decoration" minOccurs="0" maxOccurs="unbounded"/>
<xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="decoration" abstract="true" type="subDeco"/>
<xs:complexType name="subDeco" mixed="true">
<xs:sequence>
<xs:element ref="coreDecoration" minOccurs="0" maxOccurs="unbounded"/>
<xs:sequence>
</xs:complexType>
<xs:element name="coreDecoration" abstract="true" type="xs:string/>
<xs:element name="strong" substitutionGroup="decoration"/>
<xs:element name="quote" substitutionGroup="decoration"/>
<xs:element name="emph" substitutionGroup="coreDecoration"/>
in order to validate things such as
<para>This is a review of <strong>The <emph>Last</emph>
Emperor</strong>, in which we find the lead character saying <quote>I
<emph>will</emph> go to the ball!</quote>.</para>
there's nothing particular semantic about the use substitution group,
except in so far as one expects semantics to follow syntax to some
extent in _any_ coherent document type.
I'm just using the s-g mechanism to capture common patterning of the
relevant elements, and to make it easy to extend my document type by
adding more decorations in the future.
ht
--
Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
Half-time member of W3C Team
2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]
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