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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/>



<a NAME="E1:39"/>
<font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K7">'&amp;'</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K6">'not'</a> <font color="Maroon">c<sub>2</sub></font></span>)</span> <a href="zf_lang.html#R1">is_immediate_constituent_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K11">=&gt;</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="E2:39"/><i><font color="Green">E12</font></i>: 
<font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K7">'&amp;'</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K6">'not'</a> <font color="Maroon">c<sub>2</sub></font></span>)</span> <a href="zf_lang.html#R3">is_proper_subformula_of</a> <font color="Maroon">c<sub>1</sub></font> <a href="zf_lang.html#K11">=&gt;</a> <font color="Maroon">c<sub>2</sub></font>
 <b>by </b><i><a class="ref" href="zf_lang.html#T82">ZF_LANG:82</a></i>;<br/>

<a NAME="E3:39"/>
( <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> <span class="p1">(<span class="default"><a href="zf_lang.html#K6">'not'</a> <font color="Maroon">c<sub>2</sub></font></span>)</span> &amp;  <a href="zf_lang.html#K6">'not'</a> <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> <span class="p1">(<span class="default"><a href="zf_lang.html#K6">'not'</a> <font color="Maroon">c<sub>2</sub></font></span>)</span> )
 
<b>by </b><i><a class="ref" href="zf_lang.html#D39">ZF_LANG:def 39</a></i>;<br/>
<b>hence </b><a NAME="E4:39"/><i><font color="Green">E13</font></i>: 
( <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> <a href="zf_lang.html#K11">=&gt;</a> <font color="Maroon">c<sub>2</sub></font> &amp;  <a href="zf_lang.html#K6">'not'</a> <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> <a href="zf_lang.html#K11">=&gt;</a> <font color="Maroon">c<sub>2</sub></font> )
 <b>by </b><i><a class="txt" href="zf_lang1.html#E2:39"><i><font color="Green">E12</font></i></a>, <a class="ref" href="zf_lang1.html#T44" target="_self">Th44</a></i>;<br/>

<a NAME="E5:39"/>
<font color="Maroon">c<sub>2</sub></font> <a href="zf_lang.html#R1">is_immediate_constituent_of</a>  <a href="zf_lang.html#K6">'not'</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="E6:39"/>
<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> <a href="zf_lang.html#K11">=&gt;</a> <font color="Maroon">c<sub>2</sub></font>
 <b>by </b><i><a class="txt" href="zf_lang1.html#E4:39"><i><font color="Green">E13</font></i></a>, <a class="ref" href="zf_lang1.html#T44" target="_self">Th44</a></i>;<br/>


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