<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>