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<b>let </b><font color="Maroon">F</font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/><b>let </b><font color="Maroon">H</font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>



<b>assume </b><a NAME="E1:99"/><i><font color="Green">E31</font></i>: 
<font color="Maroon">F</font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">H</font>
 ;<br/>

<b>let </b><font color="Maroon">a</font> be    <a href="hidden.html#M1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a><br/>

<b>assume </b><a NAME="E2:99"/>
<font color="Maroon">a</font> <a href="hidden.html#R2">in</a>  <a href="zf_lang.html#K29" target="_self">Subformulae</a> <font color="Maroon">F</font>
 ;<br/>

<b>then consider </b><font color="Maroon">F1</font> being   <a href="zf_lang.html#NM2">ZF-formula</a><b> such that </b><br/><a NAME="E4:99"/><i><font color="Green">E32</font></i>: 
( <font color="Maroon">F1</font> <a href="hidden.html#R1">=</a> <font color="Maroon">a</font> &amp; <font color="Maroon">F1</font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">F</font> )
 <b>by </b><i><a class="ref" href="zf_lang.html#T34" target="_self">Th34</a></i>;<br/>
<a NAME="E5:99"/>
<font color="Maroon">F1</font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">H</font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T36" target="_self">Th36</a>, <a class="ref" href="zf_lang.html#T37" target="_self">Th37</a>, <a class="ref" href="zf_lang.html#T25" target="_self">Th25</a></i>;<br/>
<b>hence </b><a NAME="E6:99"/>
<font color="Maroon">a</font> <a href="hidden.html#R2">in</a>  <a href="zf_lang.html#K29" target="_self">Subformulae</a> <font color="Maroon">H</font>
 <b>by </b><i><a class="ref" href="zf_lang.html#T37" target="_self">Th37</a>, <a class="ref" href="zf_lang.html#T34" target="_self">Th34</a></i>;<br/>


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