<doc>
  <!-- at least one present -->
  <z-o>
    <b/>
  </z-o>
  <o-o>
    <c/>
  </o-o>
  <z-u>
    <d/>
  </z-u>
  <o-u>
    <e/>
  </o-u>
  <z-3>
    <f/>
  </z-3>
  <o-3>
    <g/>
  </o-3>
  <!-- more than one present when allowed -->
  <z-o>
    <b/>
  </z-o>
  <o-o>
    <c/>
  </o-o>
  <z-u>
    <d/>
    <d/>
    <d/>
  </z-u>
  <o-u>
    <e/>
    <e/>
    <e/>
  </o-u>
  <z-3>
    <f/>
    <f/>
    <f/>
  </z-3>
  <o-3>
    <g/>
    <g/>
    <g/>
  </o-3>
  <!-- min 0 are not present -->
  <z-o/>
  <o-o>
    <c/>
  </o-o>
  <z-u/>
  <o-u>
    <e/>
  </o-u>
  <z-3/>
  <o-3>
    <g/>
  </o-3>
</doc>