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

<b>assume </b><b>that </b><br/><a NAME="E1:7"/><i><font color="Green">E5</font></i>: 
<font color="Maroon">X</font> is <a href="xboole_0.html#V1">empty</a>
 <b>and </b><br/><a NAME="E2:7"/><i><font color="Green">E7</font></i>: 
<font color="Maroon">X</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">Y</font>
 ;<br/>

<a NAME="E3:7"/>
<font color="Maroon">X</font> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 
<b>by </b><i>, <a class="ref" href="xboole_0.html#D5">XBOOLE_0:def 5</a></i>;<br/>
<b>hence </b><a NAME="E4:7"/>
not <font color="Maroon">Y</font> is <a href="xboole_0.html#V1">empty</a>
 <b>by </b><i>, <a class="ref" href="xboole_0.html#D5">XBOOLE_0:def 5</a></i>;<br/>


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