Using xs:all question

Hi,

I want to define an element <a> that allows <b> or <c> in any order. Elements <b> and <c> are both optional, but <a> cannot be empty (requires at least one of <b> or <c>).

I thought this would work, but no joy (insists that both <b> and <c> are required) with Arbortext Editor:

  <xs:element name="a">
    <xs:complexType>
      <xs:all minOccurs="0">
        <xs:element ref="b"/>
        <xs:element ref="c"/>
      </xs:all>
    </xs:complexType>
  </xs:element>

Is there an easy way to do what I want using <xs:all>?  I know how to do it otherwise, but <xs:all> seems much more elegant.

Dave