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<b>let </b><font color="Maroon">T</font> be    <a href="pre_topc.html#L1">TopStruct</a> ;<br/><b>let </b><font color="Maroon">F</font> be   <a href="struct_0.html#NM3">Subset-Family</a> of <font color="Maroon">T</font>;<br/>



<a NAME="E1:11"/>
<font color="Maroon">F</font> <a href="hidden.html#R1">=</a>  <a href="setfam_1.html#K7">COMPLEMENT</a> <span class="p1">(<span class="default"><a href="setfam_1.html#K7">COMPLEMENT</a> <font color="Maroon">F</font></span>)</span>
 
;<br/>
<b>hence </b><a NAME="E2:11"/>
( <font color="Maroon">F</font> is <a href="tops_2.html#V1" target="_self">open</a> iff  <a href="setfam_1.html#K7">COMPLEMENT</a> <font color="Maroon">F</font> is <a href="tops_2.html#V2" target="_self">closed</a> )
 <b>by </b><i/>;<br/>


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