RE: optional, but at least one required

> XSD1.1 is looking at adding an xs:assert schema directive 
> that includes a set of xpath expressions that a construct 
> (sequence/choice/etc.) must satisfy.  I would be interested 
> to know how complicated such an expression for this use-case 
> would be if anyone cared to submit an example.

You can write it as

  <xs:sequence>
    <xs:element ref="a" minOccurs="0"/>
    <xs:element ref="b" minOccurs="0"/>
  </xs:sequence>
  <xs:assert test="a|b"/>

Michael Kay
http://www.saxonica.com/