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<b>let </b><font color="Maroon">c<sub>1</sub></font> be  non <a href="xboole_0.html#V1">empty</a>  <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">c<sub>2</sub></font>, <font color="Maroon">c<sub>3</sub></font> be   <a href="subset_1.html#NM2">Subset</a> of <a href="zf_lang.html#K9">WFF</a> ;<br/>





<b>assume </b><a NAME="E1:6"/><i><font color="Green">E4</font></i>: 
( <font color="Maroon">c<sub>2</sub></font> <a href="tarski.html#R1">c=</a> <font color="Maroon">c<sub>3</sub></font> &amp; <font color="Maroon">c<sub>1</sub></font> <a href="zfrefle1.html#R1" target="_self">|=</a> <font color="Maroon">c<sub>3</sub></font> )
 ;<br/>

<b>let </b><font color="Maroon">c<sub>4</sub></font> be   <a href="zf_lang.html#NM2">ZF-formula</a>; <i><font color="Red">:: according to </font></i><a class="ref" href="zfrefle1.html#D1" target="_self">ZFREFLE1:def 1</a><br/>

<b>assume </b><a NAME="E2:6"/>
<font color="Maroon">c<sub>4</sub></font> <a href="hidden.html#R2">in</a> <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<b>hence </b><a NAME="E3:6"/>
<font color="Maroon">c<sub>1</sub></font> <a href="zf_model.html#R2">|=</a> <font color="Maroon">c<sub>4</sub></font>
 <b>by </b><i><a class="txt" href="zfrefle1.html#E1:6"><i><font color="Green">E4</font></i></a>, <a class="ref" href="zfrefle1.html#D1" target="_self">Def1</a></i>;<br/>


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