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<b>let </b><font color="Maroon">c<sub>1</sub></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="E1:6_5_1"/>
( <font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R2">in</a>  <a href="numbers.html#K5">NAT</a>  implies <font color="Maroon">c<sub>1</sub></font> is <a href="ordinal1.html#V7">natural</a> )
 <b>by </b><i><a class="ref" href="ordinal1.html#D13">ORDINAL1:def 13</a></i>;<br/>


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