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<b>let </b><font color="Maroon">Y</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">SFX</font> be    <a href="hidden.html#M1">set</a> ;<br/>



<b>assume </b><a NAME="E1:19"/><i><font color="Green">E17</font></i>: 
for <font color="Olive">X</font> being   <a href="hidden.html#M1">set</a>   st <font color="Olive">X</font> <a href="hidden.html#R2">in</a> <font color="Maroon">SFX</font> holds <br/> ex <font color="Olive">Z</font> being   <a href="hidden.html#M1">set</a>  st <br/>( <font color="Olive">Z</font> <a href="hidden.html#R2">in</a> <span class="p1"><a href="tarski.html#K1">{</a><span class="default"><font color="Maroon">Y</font></span><a href="tarski.html#K1">}</a></span> &amp; <font color="Olive">X</font> <a href="tarski.html#R1">c=</a> <font color="Olive">Z</font> )
 ; <i><font color="Red">:: according to </font></i><a class="ref" href="setfam_1.html#D2" target="_self">SETFAM_1:def 2</a><br/>

<b>let </b><font color="Maroon">X</font> be    <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><a NAME="E2:19"/>
<font color="Maroon">X</font> <a href="hidden.html#R2">in</a> <font color="Maroon">SFX</font>
 ;<br/>

<a NAME="E3:19"/><b>then </b>
 ex <font color="Olive">Z</font> being   <a href="hidden.html#M1">set</a>  st <br/>( <font color="Olive">Z</font> <a href="hidden.html#R2">in</a> <span class="p1"><a href="tarski.html#K1">{</a><span class="default"><font color="Maroon">Y</font></span><a href="tarski.html#K1">}</a></span> &amp; <font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Olive">Z</font> )
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E4:19"/>
<font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Y</font>
 <b>by </b><i><a class="ref" href="tarski.html#D1">TARSKI:def 1</a></i>;<br/>


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