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<b>let </b><font color="Maroon">TS</font> be   <a href="pre_topc.html#NM1">TopSpace</a>;<br/><b>let </b><font color="Maroon">P</font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">TS</font>;<br/><b>let </b><font color="Maroon">Q</font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">TS</font>;<br/>





<a NAME="E1:108"/><i><font color="Green">E21</font></i>: 
<span class="p1">(<span class="default"><font color="Maroon">P</font> <a href="subset_1.html#K3">`</a> </span>)</span> <a href="subset_1.html#K4">\/</a> <span class="p1">(<span class="default"><font color="Maroon">Q</font> <a href="subset_1.html#K3">`</a> </span>)</span> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><font color="Maroon">P</font> <a href="subset_1.html#K9">/\</a> <font color="Maroon">Q</font></span>)</span> <a href="subset_1.html#K3">`</a> 
 
<b>by </b><i><a class="ref" href="xboole_1.html#T54">XBOOLE_1:54</a></i>;<br/>
<b>assume </b><a NAME="E2:108"/>
( <font color="Maroon">P</font> is <a href="tops_1.html#V6" target="_self">open_condensed</a> &amp; <font color="Maroon">Q</font> is <a href="tops_1.html#V6" target="_self">open_condensed</a> )
 ;<br/>

<a NAME="E3:108"/><b>then </b>
( <font color="Maroon">P</font> <a href="subset_1.html#K3">`</a>  is <a href="tops_1.html#V5" target="_self">closed_condensed</a> &amp; <font color="Maroon">Q</font> <a href="subset_1.html#K3">`</a>  is <a href="tops_1.html#V5" target="_self">closed_condensed</a> )
 
<b>by </b><i><a class="ref" href="tops_1.html#T58" target="_self">Th58</a></i>;<br/>
<a NAME="E4:108"/><b>then </b>
<span class="p1">(<span class="default"><font color="Maroon">P</font> <a href="subset_1.html#K3">`</a> </span>)</span> <a href="subset_1.html#K4">\/</a> <span class="p1">(<span class="default"><font color="Maroon">Q</font> <a href="subset_1.html#K3">`</a> </span>)</span> is <a href="tops_1.html#V5" target="_self">closed_condensed</a>
 
<b>by </b><i><a class="ref" href="tops_1.html#T65" target="_self">Th65</a></i>;<br/>
<b>hence </b><a NAME="E5:108"/>
<font color="Maroon">P</font> <a href="subset_1.html#K9">/\</a> <font color="Maroon">Q</font> is <a href="tops_1.html#V6" target="_self">open_condensed</a>
 <b>by </b><i><a class="ref" href="tops_1.html#T66" target="_self">Th66</a>, <a class="ref" href="tops_1.html#T58" target="_self">Th58</a></i>;<br/>


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