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<div class="add">

<b>let </b><font color="Maroon">p</font> be   <a href="qc_lang1.html#NM8">QC-formula</a>;<br/>

<b>thus </b><a NAME="E1:27"/>
( <font color="Maroon">p</font> is <a href="qc_lang1.html#V6">closed</a> implies  <a href="qc_lang1.html#K12">'not'</a> <font color="Maroon">p</font> is <a href="qc_lang1.html#V6">closed</a> )
 <div><a class="txt" onclick="hs2(this)" href="javascript:()" title="27_1"><b>proof </b></a><div class="add">

<b>assume </b><a NAME="E1:27_1"/>
 <a href="qc_lang1.html#K23">still_not-bound_in</a> <font color="Maroon">p</font> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 ; <i><font color="Red">:: according to </font></i><a class="ref" href="qc_lang1.html#D30">QC_LANG1:def 30</a><br/>

<b>hence </b><a NAME="E2:27_1"/>
 <a href="qc_lang1.html#K23">still_not-bound_in</a> <span class="p1">(<span class="default"><a href="qc_lang1.html#K12">'not'</a> <font color="Maroon">p</font></span>)</span> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 <b>by </b><i/>; <i><font color="Red">:: according to </font></i><a class="ref" href="qc_lang1.html#D30">QC_LANG1:def 30</a><br/>


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

<b>assume </b><a NAME="E2:27"/>
 <a href="qc_lang1.html#K23">still_not-bound_in</a> <span class="p1">(<span class="default"><a href="qc_lang1.html#K12">'not'</a> <font color="Maroon">p</font></span>)</span> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 ; <i><font color="Red">:: according to </font></i><a class="ref" href="qc_lang1.html#D30">QC_LANG1:def 30</a><br/>

<b>hence </b><a NAME="E3:27"/>
 <a href="qc_lang1.html#K23">still_not-bound_in</a> <font color="Maroon">p</font> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a> 
 <b>by </b><i/>; <i><font color="Red">:: according to </font></i><a class="ref" href="qc_lang1.html#D30">QC_LANG1:def 30</a><br/>


</div>
