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<b>let </b><font color="Maroon">A</font> be  non <a href="xboole_0.html#V1">empty</a> <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">Y</font>;<br/>

<b>assume </b><a NAME="E1:21_1_1"/>
<font color="Maroon">A</font> is <a href="realset1.html#V1">trivial</a>
 ;<br/>

<b>then consider </b><font color="Maroon">s</font> being    <a href="subset_1.html#M2">Element</a> of <font color="Maroon">A</font><b> such that </b><br/><a NAME="E3:21_1_1"/><i><font color="Green">E17</font></i>: 
<font color="Maroon">A</font> <a href="hidden.html#R1">=</a> <span class="p1"><a href="domain_1.html#K6">{</a><span class="default"><font color="Maroon">s</font></span><a href="domain_1.html#K6">}</a></span>
 <b>by </b><i/>;<br/>
<b>thus </b><a NAME="E4:21_1_1"/>
<font color="Maroon">A</font> is <a href="tex_2.html#V1" target="_self">proper</a>
 <b>by </b><i>, </i>;<br/>


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