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

<b>assume </b><a NAME="E1:93"/><i><font color="Green">E18</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">F</font> holds <br/><font color="Olive">X</font> is <a href="membered.html#V5" target="_self">natural-membered</a>
 ;<br/>

<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="membered.html#D5" target="_self">MEMBERED:def 5</a><br/>

<b>assume </b><a NAME="E2:93"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="tarski.html#K3">union</a> <font color="Maroon">F</font>
 ;<br/>

<b>then consider </b><font color="Maroon">X</font> being    <a href="hidden.html#M1">set</a> <b> such that </b><br/><a NAME="E4:93"/><i><font color="Green">E19</font></i>: 
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font>
 <b>and </b><br/><a NAME="E5:93"/><i><font color="Green">E20</font></i>: 
<font color="Maroon">X</font> <a href="hidden.html#R2">in</a> <font color="Maroon">F</font>
 <b>by </b><i><a class="ref" href="tarski.html#D4">TARSKI:def 4</a></i>;<br/>
<a NAME="E6:93"/>
<font color="Maroon">X</font> is <a href="membered.html#V5" target="_self">natural-membered</a>
 
<b>by </b><i>, <a class="ref" href="membered.html#T2" target="_self">Th2</a></i>;<br/>
<b>hence </b><a NAME="E7:93"/>
<font color="Maroon">x</font> is <a href="ordinal1.html#V7">natural</a>
 <b>by </b><i><a class="ref" href="membered.html#T1" target="_self">Th1</a>, </i>;<br/>


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