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<b>let </b><font color="Maroon">x</font> be    <a href="hidden.html#M1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a><br/>

<b>assume </b><a NAME="E1:19_1"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <font color="Maroon">F<sub>3</sub></font>(<font color="Olive">s</font>,<font color="Olive">t</font>) where <font color="Olive">s</font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>1</sub></font>(), <font color="Olive">t</font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>2</sub></font>() : <font color="Maroon">P<sub>1</sub></font>[<font color="Olive">s</font>,<font color="Olive">t</font>] </span>}</span> 
 ;<br/>

<b>then consider </b><font color="Maroon">s</font> being    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>1</sub></font>(), <font color="Maroon">t</font> being    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>2</sub></font>()<b> such that </b><br/><a NAME="E3:19_1"/><i><font color="Green">E18</font></i>: 
<font color="Maroon">x</font> <a href="hidden.html#R1">=</a> <font color="Maroon">F<sub>3</sub></font>(<font color="Maroon">s</font>,<font color="Maroon">t</font>)
 <b>and </b><br/><a NAME="E4:19_1"/><i><font color="Green">E19</font></i>: 
<font color="Maroon">P<sub>1</sub></font>[<font color="Maroon">s</font>,<font color="Maroon">t</font>]
 ;<br/>
<a NAME="E5:19_1"/>
 ex <font color="Olive">s'</font> being   <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>1</sub></font>() st <br/>( <font color="Maroon">P<sub>2</sub></font>[<font color="Olive">s'</font>,<font color="Maroon">t</font>] &amp; <font color="Maroon">F<sub>3</sub></font>(<font color="Maroon">s</font>,<font color="Maroon">t</font>) <a href="hidden.html#R1">=</a> <font color="Maroon">F<sub>3</sub></font>(<font color="Olive">s'</font>,<font color="Maroon">t</font>) )
 
<b>by </b><i>, </i>;<br/>
<b>hence </b><a NAME="E6:19_1"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <font color="Maroon">F<sub>3</sub></font>(<font color="Olive">s'</font>,<font color="Olive">t'</font>) where <font color="Olive">s'</font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>1</sub></font>(), <font color="Olive">t'</font> is    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">F<sub>2</sub></font>() : <font color="Maroon">P<sub>2</sub></font>[<font color="Olive">s'</font>,<font color="Olive">t'</font>] </span>}</span> 
 <b>by </b><i/>;<br/>


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