butes specify the attributes that all instances of the class <em>must</em> contain.</td></tr>
<tr><td style="text-align: left"><code>mayContain</code>, <code>systemMayContain</code></td><td style="text-align: center">Unicode String</td><td style="text-align: right">This pair of multi-valued attributes specify <em>optional</em> attributes that instances of the class may or may not have.</td></tr>
<tr><td style="text-align: left"><code>possSuperiors</code>, <code>systemPossSuperiors</code></td><td style="text