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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/>



<b>assume </b><a NAME="E1:42"/>
( <font color="Maroon">X</font> is <a href="card_4.html#V1" target="_self">countable</a> &amp; <font color="Maroon">Y</font> is <a href="card_4.html#V1" target="_self">countable</a> )
 ;<br/>

<a NAME="E2:42"/><b>then </b>
( <font color="Maroon">X</font> <a href="subset_1.html#K6">\</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="subset_1.html#K6">\</a> <font color="Maroon">X</font> is <a href="card_4.html#V1" target="_self">countable</a> )
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E3:42"/>
<font color="Maroon">X</font> <a href="xboole_0.html#K5">\+\</a> <font color="Maroon">Y</font> is <a href="card_4.html#V1" target="_self">countable</a>
 <b>by </b><i/>;<br/>


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