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<b>let </b><font color="Maroon">E</font> be    <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><a NAME="E1:46"/><i><font color="Green">E20</font></i>: 
<font color="Maroon">E</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a>  <a href="xboole_0.html#K1">{}</a> 
 ;<br/>

<b>let </b><font color="Maroon">B</font> be   <a href="subset_1.html#NM2">Subset</a> of <font color="Maroon">E</font>;<br/><b>let </b><font color="Maroon">x</font> be    <a href="subset_1.html#M1" target="_self">Element</a> of <font color="Maroon">E</font>;<br/>



<b>assume </b><a NAME="E2:46"/><i><font color="Green">E21</font></i>: 
not <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">B</font>
 ;<br/>

<a NAME="E3:46"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">E</font>
 
<b>by </b><i>, </i>;<br/>
<b>hence </b><a NAME="E4:46"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">B</font> <a href="subset_1.html#K3" target="_self">`</a> 
 <b>by </b><i>, <a class="ref" href="xboole_0.html#D4">XBOOLE_0:def 4</a></i>;<br/>


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