ACTION ITEM: rework general section about extending/customizing schemas (http://www.w3.org/International/its/track/actions/1)

Hi there,

Here's an modified version of the draft. The modifications should address the feedback which was sent.

Cheers,
Christian
===

The "How to do this" parts of this document often contain statements related
to schema creation or modification. The statements pertain to one of the
following state-of-affairs:

1. creating a new schema
2. modifying an existing schema 

The following aspects may need to be taken into account when working on both
of these topics:

1. Think twice before creating your own schema. Consider strongly existing
formats such as DITA, DocBook, Open Document Format, Office Open XML,
XML User Interface Language, Universal Business Language, ... Those formats 
have many insights 'built-in'.

2. The format itself should be carefully
checked with regard to modification capabilities. DocBook and DITA for
example come with their own set of features for adapting them to a special
need.

3. The mechanisms which you can or have to use depend on the schema
language (DTD, XSD, RelaxNG, ...). Namespace-based modularization of schemas
for example is difficult to realize for DTDs.

4. Each schema language provides ways of extending or modifying existing
schemas. XSD for example provides statements such as "import",
"include", or "redefine" as well as mechanisms such as type
substitution/derivation.

5. Some processors do not implement support for all schema language constructs.
Thus, a schema which works in one environment may not work in a different one.

6. What is possible also depends on the features of the schema which the
modification is targeting. Examples:

- An XSD "redefine" for example only is only
possible if the modified schema has been created with named types.

- If you are working with XSD, your options depend on the question whether
the schemas involved define target namespaces (techniques such as working
"chameleon" or "proxy" schemas may be considered as solutions in certain
cases).
===   

Christian Lieske
MultiLingual Technology Solutions (MLT)
SAP Language Services (SLS)
SAP Globalization Services
SAP AG
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Germany
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christian.lieske@sap.com
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