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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>let </b><font color="Maroon">c<sub>3</sub></font> be  non <a href="xboole_0.html#V1">empty</a> <a href="finsub_1.html#V4" target="_self">preBoolean</a>  <a href="hidden.html#M1">set</a> ;<br/>





<b>assume </b><a NAME="E1:8"/>
( <font color="Maroon">c<sub>1</sub></font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>3</sub></font> &amp; <font color="Maroon">c<sub>2</sub></font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>3</sub></font> )
 ;<br/>

<b>then reconsider </b><font color="Maroon">c<sub>4</sub></font> = <font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>5</sub></font> = <font color="Maroon">c<sub>2</sub></font> as    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>3</sub></font> ;<br/>
<a NAME="E3:8"/>
<font color="Maroon">c<sub>4</sub></font> <a href="xboole_0.html#K5">\+\</a> <font color="Maroon">c<sub>5</sub></font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><font color="Maroon">c<sub>4</sub></font> <a href="finsub_1.html#K2" target="_self">\</a> <font color="Maroon">c<sub>5</sub></font></span>)</span> <a href="finsub_1.html#K1" target="_self">\/</a> <span class="p1">(<span class="default"><font color="Maroon">c<sub>5</sub></font> <a href="finsub_1.html#K2" target="_self">\</a> <font color="Maroon">c<sub>4</sub></font></span>)</span>
 
;<br/>
<b>hence </b><a NAME="E4:8"/>
<font color="Maroon">c<sub>1</sub></font> <a href="xboole_0.html#K5">\+\</a> <font color="Maroon">c<sub>2</sub></font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>3</sub></font>
 ;<br/>


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