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<b>let </b><font color="Maroon" title="c1">T</font> be   <a href="pre_topc.html#NM1" title="PRE_TOPC:NM.1">TopSpace</a>;<br/><b>let </b><font color="Maroon" title="c2">B</font> be   <a href="struct_0.html#NM2" title="STRUCT_0:NM.2">Subset</a> of <font color="Maroon" title="c1">T</font>;<br/>



<div><b>hereby </b>
<div class="add"><b>assume </b><a NAME="E1:7_1"/>
<font color="Maroon" title="c2">B</font> is <a href="decomp_1.html#V1" title="DECOMP_1:attr.1">semi-open</a>
 ;<br/><a NAME="E2:7_1"/><b>then </b>
<font color="Maroon" title="c2">B</font> <a href="tarski.html#R1" title="TARSKI:pred.1">c=</a>  <a href="pre_topc.html#K2" title="PRE_TOPC:func.2">Cl</a> <span class="p1">(<span class="default"><a href="tops_1.html#K1" title="TOPS_1:func.1">Int</a> <font color="Maroon" title="c2">B</font></span>)</span>
 <b>by </b><i><a class="ref" href="decomp_1.html#D2" target="_self" title="DECOMP_1:def.2">Def2</a></i>;<br/><b>hence </b><a NAME="E3:7_1"/>
<font color="Maroon" title="c2">B</font> <a href="hidden.html#R1" title="HIDDEN:pred.1">=</a>  <a href="decomp_1.html#K1" title="DECOMP_1:func.1">sInt</a> <font color="Maroon" title="c2">B</font>
 <b>by </b><i><a class="ref" href="xboole_1.html#T28" title="XBOOLE_1:th.28">XBOOLE_1:28</a></i>;<br/></div>
<b>end;</b></div>

<b>assume </b><a NAME="E2:7"/>
<font color="Maroon" title="c2">B</font> <a href="hidden.html#R1" title="HIDDEN:pred.1">=</a>  <a href="decomp_1.html#K1" title="DECOMP_1:func.1">sInt</a> <font color="Maroon" title="c2">B</font>
 ;<br/>

<a NAME="E3:7"/><b>then </b>
<font color="Maroon" title="c2">B</font> <a href="tarski.html#R1" title="TARSKI:pred.1">c=</a>  <a href="pre_topc.html#K2" title="PRE_TOPC:func.2">Cl</a> <span class="p1">(<span class="default"><a href="tops_1.html#K1" title="TOPS_1:func.1">Int</a> <font color="Maroon" title="c2">B</font></span>)</span>
 
<b>by </b><i><a class="ref" href="xboole_1.html#T17" title="XBOOLE_1:th.17">XBOOLE_1:17</a></i>;<br/>
<b>hence </b><a NAME="E4:7"/>
<font color="Maroon" title="c2">B</font> is <a href="decomp_1.html#V1" title="DECOMP_1:attr.1">semi-open</a>
 <b>by </b><i><a class="ref" href="decomp_1.html#D2" target="_self" title="DECOMP_1:def.2">Def2</a></i>;<br/>


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