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<b>let </b><font color="Maroon" title="c1">A</font> be    <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ;<br/><b>let </b><font color="Maroon" title="c2">B</font> be  non <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ;<br/><b>let </b><font color="Maroon" title="c3">b</font> be    <a href="subset_1.html#M1" title="SUBSET_1:mode.1">Element</a> of <font color="Maroon" title="c2">B</font>;<br/><b>let </b><font color="Maroon" title="c4">f</font> be   <a href="functor0.html#NM1" title="FUNCTOR0:NM.1">bifunction</a> of <font color="Maroon" title="c1">A</font>,<font color="Maroon" title="c2">B</font>;<br/>







<b>assume </b><a NAME="E1:21"/><i><font color="Green" title="E18">A1</font></i>: 
<font color="Maroon" title="c4">f</font> <a href="funct_2.html#R1" title="FUNCT_2:pred.1">=</a> <span class="p1"><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">[:</a><span class="default"><font color="Maroon" title="c1">A</font>,<font color="Maroon" title="c1">A</font></span><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">:]</a></span> <a href="pboole.html#K9" title="PBOOLE:func.9">--&gt;</a> <span class="p1"><a href="tarski.html#K4" title="TARSKI:func.4">[</a><span class="default"><font color="Maroon" title="c3">b</font>,<font color="Maroon" title="c3">b</font></span><a href="tarski.html#K4" title="TARSKI:func.4">]</a></span>
 ;<br/>

<b>reconsider </b><font color="Maroon" title="c5">g</font> = <font color="Maroon" title="c1">A</font> <a href="pboole.html#K9" title="PBOOLE:func.9">--&gt;</a> <font color="Maroon" title="c3">b</font> as   <a href="funct_2.html#NM1" title="FUNCT_2:NM.1">Function</a> of <font color="Maroon" title="c1">A</font>,<font color="Maroon" title="c2">B</font> <b>by </b><i><a class="ref" href="funcop_1.html#T57" title="FUNCOP_1:th.57">FUNCOP_1:57</a></i>;<br/>
<b>thus </b><a NAME="E3:21"/>
<font color="Maroon" title="c4">f</font> is <a href="functor0.html#V1" title="FUNCTOR0:attr.1">Covariant</a>
 <div><a class="txt" onclick="hs2(this)" href="javascript:()" title="21_1"><b>proof </b></a><div class="add">

<b>take </b>
<font color="Maroon" title="c5">g</font>
; <i><font color="Red">:: according to </font></i><a class="ref" href="functor0.html#D2" target="_self" title="FUNCTOR0:def.2">FUNCTOR0:def 2</a><br/>

<b>thus </b><a NAME="E1:21_1"/>
<font color="Maroon" title="c4">f</font> <a href="relset_1.html#R2" title="RELSET_1:pred.2">=</a> <span class="p1"><a href="funct_3.html#K16" title="FUNCT_3:func.16">[:</a><span class="default"><font color="Maroon" title="c5">g</font>,<font color="Maroon" title="c5">g</font></span><a href="funct_3.html#K16" title="FUNCT_3:func.16">:]</a></span>
 <b>by </b><i><a class="txt" href="functor0.html#E1:21"><i><font color="Green" title="E18">A1</font></i></a>, <a class="ref" href="altcat_2.html#T1" title="ALTCAT_2:th.1">ALTCAT_2:1</a></i>;<br/>


</div><b>end;</b></div>

<b>take </b>
<font color="Maroon" title="c5">g</font>
; <i><font color="Red">:: according to </font></i><a class="ref" href="functor0.html#D3" target="_self" title="FUNCTOR0:def.3">FUNCTOR0:def 3</a><br/>

<a NAME="E4:21"/>
<span class="p1"><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">[:</a><span class="default"><font color="Maroon" title="c1">A</font>,<font color="Maroon" title="c1">A</font></span><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">:]</a></span> <a href="pboole.html#K9" title="PBOOLE:func.9">--&gt;</a> <span class="p1"><a href="tarski.html#K4" title="TARSKI:func.4">[</a><span class="default"><font color="Maroon" title="c3">b</font>,<font color="Maroon" title="c3">b</font></span><a href="tarski.html#K4" title="TARSKI:func.4">]</a></span> <a href="funct_2.html#R2" title="FUNCT_2:pred.2">=</a>  <a href="functor0.html#K1" title="FUNCTOR0:func.1">~</a> <span class="p1">(<span class="default"><span class="p2"><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">[:</a><span class="default"><font color="Maroon" title="c1">A</font>,<font color="Maroon" title="c1">A</font></span><a href="zfmisc_1.html#K2" title="ZFMISC_1:func.2">:]</a></span> <a href="pboole.html#K9" title="PBOOLE:func.9">--&gt;</a> <span class="p2"><a href="tarski.html#K4" title="TARSKI:func.4">[</a><span class="default"><font color="Maroon" title="c3">b</font>,<font color="Maroon" title="c3">b</font></span><a href="tarski.html#K4" title="TARSKI:func.4">]</a></span></span>)</span>
 
<b>by </b><i><a class="ref" href="functor0.html#T6" target="_self" title="FUNCTOR0:th.6">Th6</a></i>;<br/>
<b>hence </b><a NAME="E5:21"/>
<font color="Maroon" title="c4">f</font> <a href="relset_1.html#R2" title="RELSET_1:pred.2">=</a>  <a href="functor0.html#K1" title="FUNCTOR0:func.1">~</a> <span class="p1"><a href="funct_3.html#K16" title="FUNCT_3:func.16">[:</a><span class="default"><font color="Maroon" title="c5">g</font>,<font color="Maroon" title="c5">g</font></span><a href="funct_3.html#K16" title="FUNCT_3:func.16">:]</a></span>
 <b>by </b><i><a class="txt" href="functor0.html#E1:21"><i><font color="Green" title="E18">A1</font></i></a>, <a class="ref" href="altcat_2.html#T1" title="ALTCAT_2:th.1">ALTCAT_2:1</a></i>;<br/>


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