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<b>let </b><font color="Maroon">X</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">Y</font> be  <a href="membered.html#V5" target="_self">natural-membered</a>  <a href="hidden.html#M1">set</a> ;<br/>



<b>assume </b><a NAME="E1:31"/><i><font color="Green">E18</font></i>: 
<font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Y</font>
 ;<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>thus </b><a NAME="E2:31"/>
( <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font> implies <font color="Maroon">x</font> is <a href="ordinal1.html#V7">natural</a> )
 <b>by </b><i><a class="ref" href="membered.html#D5" target="_self">Def5</a>, </i>;<br/>


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