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<b>thus </b><a NAME="E1:5"/>
 <a href="xboole_0.html#K1">{}</a>  <a href="hidden.html#R1">=</a>  <a href="tarski.html#K3">union</a> <a href="xboole_0.html#K1">{}</a> 
 <b>by </b><i><a class="ref" href="zfmisc_1.html#T2">ZFMISC_1:2</a></i>; <i><font color="Red">:: according to </font></i><a class="ref" href="ordinal1.html#D6">ORDINAL1:def 6</a><br/>


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