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<b>let </b><font color="Maroon">A</font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">GX</font>;<br/>

<b>assume </b><a NAME="E1:74_1_1"/>
<font color="Maroon">A</font> is <a href="xboole_0.html#V1">empty</a>
 ;<br/>

<a NAME="E2:74_1_1"/><b>then </b>
<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="pre_topc.html#K1">{}</a> <font color="Maroon">GX</font>
 
;<br/>
<a NAME="E3:74_1_1"/><b>then </b>
<font color="Maroon">A</font> <a href="subset_1.html#K3">`</a>  <a href="hidden.html#R1">=</a>  <a href="pre_topc.html#K2">[#]</a> <font color="Maroon">GX</font>
 
;<br/>
<b>hence </b><a NAME="E4:74_1_1"/>
 <a href="pre_topc.html#K4">Cl</a> <span class="p1">(<span class="default"><font color="Maroon">A</font> <a href="subset_1.html#K3">`</a> </span>)</span> <a href="hidden.html#R1">=</a>  <a href="pre_topc.html#K2">[#]</a> <font color="Maroon">GX</font>
 <b>by </b><i/>; <i><font color="Red">:: according to </font></i><a class="ref" href="tops_1.html#D3" target="_self">TOPS_1:def 3</a>,<a class="ref" href="tops_1.html#D4" target="_self">TOPS_1:def 4</a><br/>


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