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<b>let </b><font color="Maroon" title="c2">A</font> be  non <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> <a href="struct_0.html#NM2" title="STRUCT_0:NM.2">Subset</a> of <font color="Maroon" title="c1">Y</font>;<br/>

<b>assume </b><a NAME="E1:19_2_1"/>
not <font color="Maroon" title="c2">A</font> is <a href="tex_2.html#V1" title="TEX_2:attr.1">proper</a>
 ;<br/>

<b>thus </b><a NAME="E2:19_2_1"/>
<font color="Maroon" title="c2">A</font> is <a href="realset1.html#V1" title="REALSET1:attr.1">trivial</a>
 ;<br/>


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