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<b>let </b><font color="Maroon">x</font> be    <a href="subset_1.html#M1" target="_self">Element</a> of <font color="Maroon">X</font>;<br/>

<a NAME="E1:9_1_1"/><i><font color="Green">E20</font></i>: 
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X</font>
 
<b>by </b><i/>;<br/>
<a NAME="E2:9_1_1"/>
<font color="Maroon">X</font> <a href="hidden.html#R2">in</a>  <a href="zfmisc_1.html#K1">bool</a> <font color="Maroon">D</font>
 
<b>by </b><i/>;<br/>
<a NAME="E3:9_1_1"/><b>then </b>
<font color="Maroon">X</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">D</font>
 
<b>by </b><i><a class="ref" href="zfmisc_1.html#D1">ZFMISC_1:def 1</a></i>;<br/>
<a NAME="E4:9_1_1"/><b>then </b>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">D</font>
 
<b>by </b><i>, <a class="ref" href="tarski.html#D3">TARSKI:def 3</a></i>;<br/>
<b>hence </b><a NAME="E5:9_1_1"/>
<font color="Maroon">x</font> is    <a href="subset_1.html#M1" target="_self">Element</a> of <font color="Maroon">D</font>
 <b>by </b><i/>;<br/>


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