I believe you want to use a co-occurrence constraint.You can easily 
manage this problem with RELAX NG or a combination of RELAX NG and 
Schematron. I am under a tough schedule at the moment so let me just 
point you to a couple resources. First, here is an article on the topic 
by Eddie Robertsson: http://www.xml.com/pub/a/2004/02/11/relaxtron.html. 
Also see Chapter 7 from Eric van der Vlist's book, RELAX NG: 
http://books.xmlschemata.org/relaxng/ch09s02.html.

Mike

Dan White wrote:

>I need a pointer or two about creating a schema.
>
>I have a situation where the incoming XML will have an element/attribute
>specifying one of four enumerated choices.  Depending on the value of that
>choice, the elements that follow will be different.
>
>Like this:
><foo>
>  <mode>one</mode>
>  <data1>aaa</data1>
>  <data2>bbb</data2>
></foo
>
><foo>
>  <mode>two</mode>
>  <data3>ccc</data3>
>  <data4>ddd</data4>
></foo>
>
>or
><foo mode="one">
>  <data1>aaa</data1>
>  <data2>bbb</data2>
></foo
>
><foo mode="two">
>  <data3>ccc</data3>
>  <data4>ddd</data4>
></foo>
>
>How does one set up the schema to validate the appropriate sequence of
>elements based on the value of the "controlling" element/attribute ?
>
>Thanks in advance.
>
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