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<b>consider </b><font color="Maroon" title="c2">A</font> being  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> <a href="compl_sp.html#V2" title="COMPL_SP:attr.2">open</a> <a href="struct_0.html#NM2" title="STRUCT_0:NM.2">Subset</a> of <font color="Maroon" title="c1">M</font>;<br/>
<a NAME="E2:12_2_1"/>
<span class="p1">(<span class="default"><font color="Maroon" title="c2">A</font> <a href="subset_1.html#K3" title="SUBSET_1:func.3">`</a> </span>)</span> <a href="subset_1.html#K3" title="SUBSET_1:func.3">`</a>  <a href="hidden.html#R1" title="HIDDEN:pred.1">=</a> <font color="Maroon" title="c2">A</font>
 
;<br/>
<a NAME="E3:12_2_1"/><b>then </b>
( <font color="Maroon" title="c2">A</font> <a href="subset_1.html#K3" title="SUBSET_1:func.3">`</a>  <a href="hidden.html#R1" title="HIDDEN:pred.1">=</a>  <a href="struct_0.html#K2" title="STRUCT_0:func.2">[#]</a> <font color="Maroon" title="c1">M</font> &amp; <font color="Maroon" title="c2">A</font> <a href="subset_1.html#K3" title="SUBSET_1:func.3">`</a>  is <a href="compl_sp.html#V3" title="COMPL_SP:attr.3">closed</a> )
 
<b>by </b><i><a class="ref" href="compl_sp.html#D4" target="_self" title="COMPL_SP:def.4">Def4</a></i>;<br/>
<b>hence </b><a NAME="E4:12_2_1"/>
 ex <font color="Olive">b<sub>1</sub></font> being  <a href="struct_0.html#NM2" title="STRUCT_0:NM.2">Subset</a> of <font color="Maroon" title="c1">M</font> st <br/>( <font color="Olive">b<sub>1</sub></font> is <a href="compl_sp.html#V3" title="COMPL_SP:attr.3">closed</a> &amp; not <font color="Olive">b<sub>1</sub></font> is <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> )
 ;<br/>


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