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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/>





<a NAME="E1:37"/>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R1">is_immediate_constituent_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K7">'&amp;'</a> <font color="Maroon">c<sub>2</sub></font> &amp; <font color="Maroon">c<sub>2</sub></font> <a href="zf_lang.html#R1">is_immediate_constituent_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K7">'&amp;'</a> <font color="Maroon">c<sub>2</sub></font> )
 
<b>by </b><i><a class="ref" href="zf_lang.html#D39">ZF_LANG:def 39</a></i>;<br/>
<b>hence </b><a NAME="E2:37"/>
( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K7">'&amp;'</a> <font color="Maroon">c<sub>2</sub></font> <a href="zf_lang.html#R2">is_subformula_of</a> <font color="Maroon">c<sub>3</sub></font> implies ( <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#R3">is_proper_subformula_of</a> <font color="Maroon">c<sub>3</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>3</sub></font> ) )
 <b>by </b><i><a class="ref" href="zf_lang1.html#T44" target="_self">Th44</a></i>;<br/>


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