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<b>let </b><font color="Maroon" title="c1">x</font>, <font color="Maroon" title="c2">y</font> be   <a href="zf_lang.html#NM1" title="ZF_LANG:NM.1">Variable</a>;<br/><b>let </b><font color="Maroon" title="c3">F</font> be   <a href="zf_lang.html#NM2" title="ZF_LANG:NM.2">ZF-formula</a>;<br/>





<b>assume </b><a NAME="E1:86"/>
<font color="Maroon" title="c3">F</font> <a href="zf_lang.html#R3" title="ZF_LANG:pred.3">is_proper_subformula_of</a> <font color="Maroon" title="c1">x</font> <a href="zf_lang.html#K4" title="ZF_LANG:func.4">'='</a> <font color="Maroon" title="c2">y</font>
 ;<br/>

<a NAME="E2:86"/><b>then </b>
 ex <font color="Olive" title="b1">G</font> being  <a href="zf_lang.html#NM2" title="ZF_LANG:NM.2">ZF-formula</a> st <font color="Olive" title="b1">G</font> <a href="zf_lang.html#R1" title="ZF_LANG:pred.1">is_immediate_constituent_of</a> <font color="Maroon" title="c1">x</font> <a href="zf_lang.html#K4" title="ZF_LANG:func.4">'='</a> <font color="Maroon" title="c2">y</font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T84" target="_self" title="ZF_LANG:th.84">Th84</a></i>;<br/>
<b>hence </b><a NAME="E3:86"/>
contradiction
 <b>by </b><i><a class="ref" href="zf_lang.html#T69" target="_self" title="ZF_LANG:th.69">Th69</a></i>;<br/>


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