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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">TS</font>;<br/>

<b>assume </b><a NAME="E1:36_1_1"/>
<font color="Maroon">A</font> is <a href="xboole_0.html#V1">empty</a>
 ;<br/>

<a NAME="E2:36_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">TS</font>
 
;<br/>
<b>hence </b><a NAME="E3:36_1_1"/>
( <font color="Maroon">A</font> is <a href="pre_topc.html#V3">open</a> &amp; <font color="Maroon">A</font> is <a href="pre_topc.html#V4">closed</a> )
 ;<br/>


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