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<b>let </b><font color="Maroon">x</font> be   <a href="qc_lang1.html#NM3">bound_QC-variable</a>;<br/><b>let </b><font color="Maroon">p</font> be   <a href="qc_lang1.html#NM8">QC-formula</a>;<br/>



<b>assume </b><a NAME="E1:30"/>
 <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/>

<a NAME="E2:30"/><b>then </b>
 <a href="qc_lang1.html#K23">still_not-bound_in</a> <font color="Maroon">p</font> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="tarski.html#K1">{</a><span class="default"><font color="Maroon">x</font></span><a href="tarski.html#K1">}</a></span>
 
<b>by </b><i><a class="ref" href="xboole_1.html#T2">XBOOLE_1:2</a></i>;<br/>
<b>hence </b><a NAME="E3:30"/>
 <a href="qc_lang1.html#K14">All</a> <font color="Maroon">x</font>,<font color="Maroon">p</font> is <a href="qc_lang1.html#V6">closed</a>
 <b>by </b><i/>;<br/>


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