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<b>let </b><font color="Maroon">X</font> be  <a href="finset_1.html#V1">finite</a>  <a href="hidden.html#M1">set</a> , <font color="Maroon">Y</font> be  <a href="finset_1.html#V1">finite</a>  <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><a NAME="E1:19"/>
<font color="Maroon">X</font> <a href="xboole_0.html#R1">misses</a> <font color="Maroon">Y</font>
 ;<br/>

<a NAME="E2:19"/><b>then </b>
 <a href="card_1.html#K4">card</a> <span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y</font></span>)</span> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="card_1.html#K4">card</a> <font color="Maroon">X</font></span>)</span> <a href="nat_1.html#K1">+</a> <span class="p1">(<span class="default"><a href="card_1.html#K4">card</a> <font color="Maroon">Y</font></span>)</span>
 
<b>by </b><i><a class="ref" href="card_2.html#T53">CARD_2:53</a></i>;<br/>
<b>hence </b><a NAME="E3:19"/>
( (  <a href="card_1.html#K4">card</a> <font color="Maroon">X</font> is <a href="abian.html#V1">even</a> iff  <a href="card_1.html#K4">card</a> <font color="Maroon">Y</font> is <a href="abian.html#V1">even</a> ) iff  <a href="card_1.html#K4">card</a> <span class="p1">(<span class="default"><font color="Maroon">X</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y</font></span>)</span> is <a href="abian.html#V1">even</a> )
 <b>by </b><i/>;<br/>


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