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<b>let </b><font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>2</sub></font> be    <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><a NAME="E1:101"/><i><font color="Green">E44</font></i>: 
<font color="Maroon">c<sub>1</sub></font> <a href="xboole_0.html#R2">c&lt;</a> <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<b>assume </b><a NAME="E2:101"/>
<font color="Maroon">c<sub>2</sub></font> <a href="xboole_0.html#K4">\</a> <font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 ;<br/>

<a NAME="E3:101"/><b>then </b>
<font color="Maroon">c<sub>2</sub></font> <a href="tarski.html#R1">c=</a> <font color="Maroon">c<sub>1</sub></font>
 
<b>by </b><i><a class="txt" href="xboole_1.html#E32"><i><font color="Green">Lemma17</font></i></a></i>;<br/>
<b>hence </b><a NAME="E4:101"/>
contradiction
 <b>by </b><i><a class="txt" href="xboole_1.html#E1:101"><i><font color="Green">E44</font></i></a>, <a class="ref" href="xboole_1.html#T60" target="_self">Th60</a></i>;<br/>


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