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<b>let </b><font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>2</sub></font> be   <a href="nat_1.html#NM1">Nat</a>;<br/>



<b>assume </b><a NAME="E1:31"/>
( <font color="Maroon">c<sub>1</sub></font> is <a href="abian.html#V1">even</a> &amp; <font color="Maroon">c<sub>2</sub></font> is <a href="abian.html#V1">even</a> )
 ;<br/>

<b>then reconsider </b><font color="Maroon">c<sub>3</sub></font> = <font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>4</sub></font> = <font color="Maroon">c<sub>2</sub></font> as  <a href="abian.html#V1">even</a> <a href="nat_1.html#NM1">Nat</a> ;<br/>
<a NAME="E3:31"/>
<font color="Maroon">c<sub>3</sub></font> <a href="nat_1.html#K2">*</a> <font color="Maroon">c<sub>4</sub></font> is <a href="abian.html#V1">even</a>
 
;<br/>
<b>hence </b><a NAME="E4:31"/>
<font color="Maroon">c<sub>1</sub></font> <a href="nat_1.html#K2">*</a> <font color="Maroon">c<sub>2</sub></font> is <a href="abian.html#V1">even</a>
 ;<br/>


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