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<b>let </b><font color="Maroon">c<sub>1</sub></font>, <font color="Maroon">c<sub>2</sub></font>, <font color="Maroon">c<sub>3</sub></font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/>





<b>assume </b><a NAME="E1:109"/><i><font color="Green">E85</font></i>: 
( ( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R1" target="_self">is_immediate_constituent_of</a> <font color="Maroon">c<sub>2</sub></font> or <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R3" target="_self">is_proper_subformula_of</a> <font color="Maroon">c<sub>2</sub></font> or <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>2</sub></font> ) &amp; <font color="Maroon">c<sub>2</sub></font> <a href="hidden.html#R2">in</a>  <a href="zf_lang.html#K29" target="_self">Subformulae</a> <font color="Maroon">c<sub>3</sub></font> )
 ;<br/>

<a NAME="E2:109"/><b>then </b>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R3" target="_self">is_proper_subformula_of</a> <font color="Maroon">c<sub>2</sub></font> or <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>2</sub></font> )
 
<b>by </b><i><a class="ref" href="zf_lang.html#T82" target="_self">Th82</a></i>;<br/>
<a NAME="E3:109"/><b>then </b>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R2" target="_self">is_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#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>3</sub></font> )
 
<b>by </b><i><a class="txt" href="zf_lang.html#E1:109"><i><font color="Green">E85</font></i></a>, <a class="ref" href="zf_lang.html#D41" target="_self">Def41</a>, <a class="ref" href="zf_lang.html#T100" target="_self">Th100</a></i>;<br/>
<a NAME="E4:109"/><b>then </b>
<font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R2" target="_self">is_subformula_of</a> <font color="Maroon">c<sub>3</sub></font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T86" target="_self">Th86</a></i>;<br/>
<b>hence </b><a NAME="E5:109"/>
<font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R2">in</a>  <a href="zf_lang.html#K29" target="_self">Subformulae</a> <font color="Maroon">c<sub>3</sub></font>
 <b>by </b><i><a class="ref" href="zf_lang.html#D42" target="_self">Def42</a></i>;<br/>


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