Basic XSD question

Hello,

I have a quite basic question, but I could find a solution.

How can I define in XMLSchema that an element should provide a text
value? When I validate the following XML with the XSD I get no error,
because the empty string seems to be a valid string as well.

XML:
----
<root>
	<a></a> <-- if a is <a> is included it should have an xs:string
text node
	<a></a> <-- a second <a> is also not allowed
</root>

XSD fragment:
-------------
	<xs:element name="root">
		<xs:complexType>
			<xs:choice maxOccur="1"> <-- why does it not
invalidate the second <a>?
				<xs:element name="foo" type="xs:string"
minOccurs="1" />
			</xs:choice>
		</xs:complexType>
	</xs:element>

Thanks for replies,

R.