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<b>let </b><font color="Maroon">c<sub>1</sub></font> be   <a href="relat_1.html#NM1">Relation</a>;<br/>

<b>consider </b><font color="Maroon">c<sub>2</sub></font> being    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>1</sub></font> <a href="relat_1.html#K10" target="_self">"</a> <a href="xboole_0.html#K1">{}</a> ;<br/>
<b>assume </b><a NAME="E2:161"/><i><font color="Green">E73</font></i>: 
not <font color="Maroon">c<sub>1</sub></font> <a href="relat_1.html#K10" target="_self">"</a> <a href="xboole_0.html#K1">{}</a>  <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 ;<br/>

<a NAME="E3:161"/>
( <font color="Maroon">c<sub>2</sub></font> <a href="hidden.html#R2">in</a> <font color="Maroon">c<sub>1</sub></font> <a href="relat_1.html#K10" target="_self">"</a> <a href="xboole_0.html#K1">{}</a>  iff ex <font color="Olive">b<sub>1</sub></font> being   <a href="hidden.html#M1">set</a>  st <br/>( <span class="p1"><a href="tarski.html#K4">[</a><span class="default"><font color="Maroon">c<sub>2</sub></font>,<font color="Olive">b<sub>1</sub></font></span><a href="tarski.html#K4">]</a></span> <a href="hidden.html#R2">in</a> <font color="Maroon">c<sub>1</sub></font> &amp; <font color="Olive">b<sub>1</sub></font> <a href="hidden.html#R2">in</a>  <a href="xboole_0.html#K1">{}</a>  ) )
 
<b>by </b><i><a class="ref" href="relat_1.html#D14" target="_self">Def14</a></i>;<br/>
<b>hence </b><a NAME="E4:161"/>
contradiction
 <b>by </b><i><a class="txt" href="relat_1.html#E2:161"><i><font color="Green">E73</font></i></a></i>;<br/>


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