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<b>let </b><font color="Maroon">c<sub>2</sub></font> be    <a href="subset_1.html#M1">Element</a> of <font color="Maroon">c<sub>1</sub></font>;<br/>

<a NAME="E1:60_1_1"/>
( <font color="Maroon">c<sub>1</sub></font> is <a href="xboole_0.html#V1">empty</a> or  not <font color="Maroon">c<sub>1</sub></font> is <a href="xboole_0.html#V1">empty</a> )
 
;<br/>
<b>hence </b><a NAME="E2:60_1_1"/>
<font color="Maroon">c<sub>2</sub></font> is <a href="ordinal1.html#V3">ordinal</a>
 <b>by </b><i><a class="ref" href="ordinal1.html#T23">ORDINAL1:23</a>, <a class="ref" href="subset_1.html#D2">SUBSET_1:def 2</a></i>;<br/>


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