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





<b>assume </b><a NAME="E1:106"/>
( <font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font> &amp; <font color="Maroon">Y</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font> )
 ;<br/>

<a NAME="E2:106"/><b>then </b>
( <font color="Maroon">X</font> <a href="xboole_0.html#K4">\</a> <font color="Maroon">Y</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font> &amp; <font color="Maroon">Y</font> <a href="xboole_0.html#K4">\</a> <font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font> )
 
<b>by </b><i><a class="txt" href="xboole_1.html#E58"><i><font color="Green">Lemma45</font></i></a></i>;<br/>
<a NAME="E3:106"/><b>then </b>
<span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="xboole_0.html#K4">\</a> <font color="Maroon">Y</font></span>)</span> <a href="xboole_0.html#K2">\/</a> <span class="p1">(<span class="default"><font color="Maroon">Y</font> <a href="xboole_0.html#K4">\</a> <font color="Maroon">X</font></span>)</span> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font>
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E4:106"/>
<font color="Maroon">X</font> <a href="xboole_0.html#K5">\+\</a> <font color="Maroon">Y</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">A</font>
 ;<br/>


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