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<b>let </b><font color="Maroon">H</font> be   <a href="zf_lang.html#NM2">ZF-formula</a>;<br/><b>let </b><font color="Maroon">M</font> be  non <a href="xboole_0.html#V1">empty</a>  <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">v</font> be   <a href="funct_2.html#NM1">Function</a> of  <a href="zf_lang.html#K1">VAR</a> ,<font color="Maroon">M</font>;<br/>





<b>assume </b><a NAME="E1:86"/>
<font color="Maroon">H</font> is <a href="zf_lang.html#V10">biconditional</a>
 ;<br/>

<a NAME="E2:86"/><b>then </b>
<font color="Maroon">H</font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K27">the_left_side_of</a> <font color="Maroon">H</font></span>)</span> <a href="zf_lang.html#K12">&lt;=&gt;</a> <span class="p1">(<span class="default"><a href="zf_lang.html#K28">the_right_side_of</a> <font color="Maroon">H</font></span>)</span>
 
<b>by </b><i><a class="ref" href="zf_lang.html#T67">ZF_LANG:67</a></i>;<br/>
<b>hence </b><a NAME="E3:86"/>
( <font color="Maroon">M</font>,<font color="Maroon">v</font> <a href="zf_model.html#R1">|=</a> <font color="Maroon">H</font> iff ( <font color="Maroon">M</font>,<font color="Maroon">v</font> <a href="zf_model.html#R1">|=</a>  <a href="zf_lang.html#K27">the_left_side_of</a> <font color="Maroon">H</font> iff <font color="Maroon">M</font>,<font color="Maroon">v</font> <a href="zf_model.html#R1">|=</a>  <a href="zf_lang.html#K28">the_right_side_of</a> <font color="Maroon">H</font> ) )
 <b>by </b><i><a class="ref" href="zf_model.html#T19">ZF_MODEL:19</a></i>;<br/>


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