XSD for 'at least one of'

Apologies if this is a PAQ, I can't find the answer.
How do I specify in my XSD that an element must contain at least one of it's
child elements?
 I've tried the following:

   <xs:complexType name="options">

 <xs:choice minOccurs="1" maxOccurs="unbounded">

 <xs:element name='month' type="month" minOccurs="0" />

 <xs:element name='year' type="year" minOccurs="0" />

 <xs:element name='day' type="day" minOccurs="0" />

 </xs:choice>

</xs:complexType>

but it still doesn't seem to care if 'options' is empty.


-- 
George Francis
e-mail: gfrancis1@gmail.com