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<b>let </b><font color="Maroon">A</font> be   <a href="altcat_1.html#NM4">category</a>, <font color="Maroon">B</font> be   <a href="altcat_1.html#NM4">category</a>;<br/>

<a NAME="E1:51_1_1"/>
( <font color="Maroon">A</font>,<font color="Maroon">A</font> <a href="yellow18.html#K1" target="_self">opp</a>  <a href="yellow18.html#R2" target="_self">are_opposite</a>  &amp; <font color="Maroon">B</font>,<font color="Maroon">B</font> <a href="yellow18.html#K1" target="_self">opp</a>  <a href="yellow18.html#R2" target="_self">are_opposite</a>  )
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E2:51_1_1"/>
( <font color="Maroon">A</font>,<font color="Maroon">B</font> <a href="yellow18.html#K1" target="_self">opp</a>  <a href="yellow18.html#R1" target="_self">are_equivalent</a>  implies <font color="Maroon">B</font>,<font color="Maroon">A</font> <a href="yellow18.html#K1" target="_self">opp</a>  <a href="yellow18.html#R1" target="_self">are_equivalent</a>  )
 <b>by </b><i/>;<br/>


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