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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#NM1">Variable</a>;<br/><b>let </b><font color="Maroon">c<sub>3</sub></font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>





<b>thus </b><a NAME="E1:96"/>
( <font color="Maroon">c<sub>3</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font> implies <font color="Maroon">c<sub>3</sub></font> <a href="hidden.html#R1">=</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font> )
 <div><a class="txt" onclick="hs2(this)" href="javascript:()" title="96_1"><b>proof </b></a><div class="add">

<b>assume </b><a NAME="E1:96_1"/><i><font color="Green">E78</font></i>: 
<font color="Maroon">c<sub>3</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<b>assume </b><a NAME="E2:96_1"/>
<font color="Maroon">c<sub>3</sub></font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<a NAME="E3:96_1"/><b>then </b>
<font color="Maroon">c<sub>3</sub></font> <a href="zf_lang.html#R3" target="_self">is_proper_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 
<b>by </b><i><a class="txt" href="zf_lang.html#E1:96_1"><i><font color="Green">E78</font></i></a>, <a class="ref" href="zf_lang.html#D41" target="_self">Def41</a></i>;<br/>
<a NAME="E4:96_1"/><b>then </b>
ex <font color="Olive">b<sub>1</sub></font> being  <a href="zf_lang.html#NM2">ZF-formula</a> st <font color="Olive">b<sub>1</sub></font> <a href="zf_lang.html#R1" target="_self">is_immediate_constituent_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T84" target="_self">Th84</a></i>;<br/>
<b>hence </b><a NAME="E5:96_1"/>
contradiction
 <b>by </b><i><a class="ref" href="zf_lang.html#T70" target="_self">Th70</a></i>;<br/>


</div><b>end;</b></div>

<b>assume </b><a NAME="E2:96"/>
<font color="Maroon">c<sub>3</sub></font> <a href="hidden.html#R1">=</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<b>hence </b><a NAME="E3:96"/>
<font color="Maroon">c<sub>3</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K5" target="_self">'in'</a> <font color="Maroon">c<sub>2</sub></font>
 <b>by </b><i><a class="ref" href="zf_lang.html#T79" target="_self">Th79</a></i>;<br/>


</div>
