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<div><span class="kw">theorem </span><span class="lab"><font color="Green" title="E23">Th21</font></span>: <a NAME="T21"><span class="comment"><font color="firebrick">:: CLASSES5:21</font></span><br/></a><div class="add"> for <font color="Olive" title="b1">U</font> being   <a href="classes2.html#NM1" title="CLASSES2:NM.1">Universe</a><br/>  for <font color="Olive" title="b2">X</font> being    <a href="hidden.html#M2" title="HIDDEN:mode.2">set</a>   st  <a href="tarski.html#K3" title="TARSKI:func.3">union</a> <font color="Olive" title="b2">X</font> <a href="tarski.html#R2" title="TARSKI:pred.2">in</a> <font color="Olive" title="b1">U</font> holds <br/><font color="Olive" title="b2">X</font> <a href="tarski.html#R2" title="TARSKI:pred.2">in</a> <font color="Olive" title="b1">U</font></div></div>
