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<b>take </b>
 <a href="xboole_0.html#K1">{}</a> 
;<br/>

<b>thus </b><a NAME="E1:6_1_1"/>
(  <a href="xboole_0.html#K1">{}</a>  is <a href="xboole_0.html#V1">empty</a> &amp;  <a href="xboole_0.html#K1">{}</a>  is <a href="fraenkel.html#V1">functional</a> )
 ;<br/>


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