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<b>let </b><font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>2</sub></font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>



<b>assume </b><a NAME="E1:33"/>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R3">is_proper_subformula_of</a> <font color="Maroon">c<sub>2</sub></font> &amp; <font color="Maroon">c<sub>2</sub></font> <a href="zf_lang.html#R3">is_proper_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> )
 ;<br/>

<a NAME="E2:33"/><b>then </b>
<font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R3">is_proper_subformula_of</a> <font color="Maroon">c<sub>1</sub></font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T85">ZF_LANG:85</a></i>;<br/>
<b>hence </b><a NAME="E3:33"/>
contradiction
 <b>by </b><i><a class="ref" href="zf_lang.html#D41">ZF_LANG:def 41</a></i>;<br/>


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