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<b>let </b><font color="Maroon" title="c1">X</font> be    <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ;<br/>

<b>assume </b><a NAME="E1:61_1_1"/>
not <font color="Maroon" title="c1">X</font> is <a href="setfam_1.html#V2" title="SETFAM_1:attr.2">empty-membered</a>
 ;<br/>

<a NAME="E2:61_1_1"/><b>then </b>
 ex <font color="Olive" title="b1">x</font> being non <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a>  st <font color="Olive" title="b1">x</font> <a href="hidden.html#R2" title="HIDDEN:pred.2">in</a> <font color="Maroon" title="c1">X</font>
 
<b>by </b><i><a class="ref" href="setfam_1.html#D11" target="_self" title="SETFAM_1:def.11">Def11</a></i>;<br/>
<b>hence </b><a NAME="E3:61_1_1"/>
not <font color="Maroon" title="c1">X</font> is <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>
 ;<br/>


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