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

<b>assume </b><a NAME="E1:65"/><i><font color="Green">E53</font></i>: 
<font color="Maroon">c<sub>1</sub></font> is <a href="zf_lang.html#V10" target="_self">biconditional</a>
 ;<br/>

<a NAME="E2:65"/><b>then </b>
ex <font color="Olive">b<sub>1</sub></font> being  <a href="zf_lang.html#NM2">ZF-formula</a> st <font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K27" target="_self">the_left_side_of</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> <a href="zf_lang.html#K12" target="_self">&lt;=&gt;</a> <font color="Olive">b<sub>1</sub></font>
 
<b>by </b><i><a class="ref" href="zf_lang.html#D37" target="_self">Def37</a></i>;<br/>
<b>hence </b><a NAME="E3:65"/>
<font color="Maroon">c<sub>1</sub></font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K27" target="_self">the_left_side_of</a> <font color="Maroon">c<sub>1</sub></font></span>)</span> <a href="zf_lang.html#K12" target="_self">&lt;=&gt;</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K28" target="_self">the_right_side_of</a> <font color="Maroon">c<sub>1</sub></font></span>)</span>
 <b>by </b><i><a class="txt" href="zf_lang.html#E1:65"><i><font color="Green">E53</font></i></a>, <a class="ref" href="zf_lang.html#D38" target="_self">Def38</a></i>;<br/>


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