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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="altcat_1.html#NM4">category</a>;<br/>

<b>assume </b><a NAME="E1:21"/><i><font color="Green">E6</font></i>: 
<font color="Maroon">c<sub>1</sub></font>,<font color="Maroon">c<sub>2</sub></font> <a href="functor0.html#R1">are_isomorphic</a> 
 ;<br/>

<b>consider </b><font color="Maroon">c<sub>3</sub></font> being    <a href="functor0.html#M2">Functor</a> of <font color="Maroon">c<sub>1</sub></font>,<font color="Maroon">c<sub>2</sub></font><b> such that </b><br/><a NAME="E3:21"/><i><font color="Green">E7</font></i>: 
( <font color="Maroon">c<sub>3</sub></font> is <a href="functor0.html#V21">bijective</a> &amp; <font color="Maroon">c<sub>3</sub></font> is <a href="functor0.html#V15">covariant</a> )
 <b>by </b><i><a class="txt" href="yellow18.html#E1:21"><i><font color="Green">E6</font></i></a>, <a class="ref" href="functor0.html#D40">FUNCTOR0:def 40</a></i>;<br/>
<b>reconsider </b><font color="Maroon">c<sub>4</sub></font> = <font color="Maroon">c<sub>3</sub></font> as  <a href="functor0.html#V15">covariant</a>  <a href="functor0.html#M2">Functor</a> of <font color="Maroon">c<sub>1</sub></font>,<font color="Maroon">c<sub>2</sub></font> <b>by </b><i><a class="txt" href="yellow18.html#E3:21"><i><font color="Green">E7</font></i></a></i>;<br/>
<b>consider </b><font color="Maroon">c<sub>5</sub></font> being    <a href="functor0.html#M2">Functor</a> of <font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>1</sub></font><b> such that </b><br/><a NAME="E6:21"/><i><font color="Green">E8</font></i>: 
( <font color="Maroon">c<sub>5</sub></font> <a href="hidden.html#R1">=</a> <font color="Maroon">c<sub>4</sub></font> <a href="functor0.html#K15">"</a>  &amp; <font color="Maroon">c<sub>5</sub></font> is <a href="functor0.html#V21">bijective</a> &amp; <font color="Maroon">c<sub>5</sub></font> is <a href="functor0.html#V15">covariant</a> )
 <b>by </b><i><a class="txt" href="yellow18.html#E3:21"><i><font color="Green">E7</font></i></a>, <a class="ref" href="functor0.html#T49">FUNCTOR0:49</a></i>;<br/>
<b>reconsider </b><font color="Maroon">c<sub>6</sub></font> = <font color="Maroon">c<sub>5</sub></font> as  <a href="functor0.html#V15">covariant</a>  <a href="functor0.html#M2">Functor</a> of <font color="Maroon">c<sub>2</sub></font>,<font color="Maroon">c<sub>1</sub></font> <b>by </b><i><a class="txt" href="yellow18.html#E6:21"><i><font color="Green">E8</font></i></a></i>;<br/>
<b>take </b>
<font color="Maroon">c<sub>4</sub></font>
; <i><font color="Red">:: according to </font></i><a class="ref" href="yellow18.html#D2" target="_self">YELLOW18:def 2</a><br/>

<b>take </b>
<font color="Maroon">c<sub>6</sub></font>
;<br/>

<b>thus </b><a NAME="E8:21"/>
( <font color="Maroon">c<sub>6</sub></font> <a href="functor3.html#K1">*</a> <font color="Maroon">c<sub>4</sub></font>, <a href="functor0.html#K12">id</a> <font color="Maroon">c<sub>1</sub></font> <a href="functor3.html#R1">are_naturally_equivalent</a>  &amp; <font color="Maroon">c<sub>4</sub></font> <a href="functor3.html#K1">*</a> <font color="Maroon">c<sub>6</sub></font>, <a href="functor0.html#K12">id</a> <font color="Maroon">c<sub>2</sub></font> <a href="functor3.html#R1">are_naturally_equivalent</a>  )
 <b>by </b><i><a class="txt" href="yellow18.html#E3:21"><i><font color="Green">E7</font></i></a>, <a class="txt" href="yellow18.html#E6:21"><i><font color="Green">E8</font></i></a>, <a class="ref" href="functor1.html#T21">FUNCTOR1:21</a>, <a class="ref" href="functor1.html#T22">FUNCTOR1:22</a></i>;<br/>


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