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<b>consider </b><font color="Maroon" title="c1">X</font> being  non <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> <a href="membered.html#V6" title="MEMBERED:attr.6">natural-membered</a>  <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ;<br/>
<b>take </b>
<font color="Maroon" title="c1">X</font>
;<br/>

<b>thus </b><a NAME="E2:116_1_1"/>
( not <font color="Maroon" title="c1">X</font> is <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> &amp; <font color="Maroon" title="c1">X</font> is <a href="membered.html#V6" title="MEMBERED:attr.6">natural-membered</a> )
 ;<br/>


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