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<b>let </b><font color="Maroon">X</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">Y</font> be    <a href="hidden.html#M1">set</a> ;<br/>



<a NAME="E1:41"/>
( <font color="Maroon">X</font> <a href="xboole_0.html#K3">/\</a> <font color="Maroon">Y</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X</font> &amp; <font color="Maroon">Y</font> <a href="xboole_0.html#K3">/\</a> <font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X</font> )
 
<b>by </b><i><a class="ref" href="xboole_1.html#T17">XBOOLE_1:17</a></i>;<br/>
<b>hence </b><a NAME="E2:41"/>
( <font color="Maroon">X</font> is <a href="card_4.html#V1" target="_self">countable</a> implies ( <font color="Maroon">X</font> <a href="xboole_0.html#K3">/\</a> <font color="Maroon">Y</font> is <a href="card_4.html#V1" target="_self">countable</a> &amp; <font color="Maroon">Y</font> <a href="xboole_0.html#K3">/\</a> <font color="Maroon">X</font> is <a href="card_4.html#V1" target="_self">countable</a> ) )
 <b>by </b><i/>;<br/>


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