XML schema -- specify a relationship between attributes?

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Hey All -

I hope this is an appropriate place for this question.

I'm stuck while trying to write a schema, hoping someone can provide me
with a flash of inspiration.

One of the elements I'm specifying has two attributes, both of which are
optional. I want to express in the schema that there's a relationship
between the two -- that the second attribute is optional only if the
first one is not specified. ie:

<myElement /> <!-- valid -->
<myElement name="A" /> <!-- valid -->
<myElement myattr="true" /> <!-- invalid because name isn't specified -->
<myElement myattr="true" name="A" /> <!-- valid -->

So far my schema looks like this:

<element name="myElement">
	<complexType>
		<attribute name="name" type="string" use="optional" />
		<attribute name="myattr" type="boolean" use="optional" default="false" />
	</complexType>
</element>

That captures the default values and the optional nature of the two
elements, but it doesn't capture the relationship between the two. Is
there any xsd construct I can use to express the relationship? I know
how to do it with elements (xsd:group, xsd:choice) but not with attributes.

Thanks for any thoughts.

Stewart
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