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<b>let </b><font color="Maroon">x</font> be   <a href="zf_lang.html#NM1">Variable</a>;<br/><b>let </b><font color="Maroon">y</font> be   <a href="zf_lang.html#NM1">Variable</a>;<br/><b>let </b><font color="Maroon">F</font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>





<b>assume </b><a NAME="E1:86"/>
<font color="Maroon">F</font> <a href="zf_lang.html#R3" target="_self">is_proper_subformula_of</a> <font color="Maroon">x</font> <a href="zf_lang.html#K4" target="_self">'='</a> <font color="Maroon">y</font>
 ;<br/>

<a NAME="E2:86"/><b>then </b>
 ex <font color="Olive">G</font> being  <a href="zf_lang.html#NM2">ZF-formula</a> st <font color="Olive">G</font> <a href="zf_lang.html#R1" target="_self">is_immediate_constituent_of</a> <font color="Maroon">x</font> <a href="zf_lang.html#K4" target="_self">'='</a> <font color="Maroon">y</font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#D22" target="_self">Def22</a></i>;<br/>
<b>hence </b><a NAME="E3:86"/>
contradiction
 <b>by </b><i/>;<br/>


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