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<b>let </b><font color="Maroon">D1</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">D2</font> be    <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><b>that </b><br/><a NAME="E1:5_1_2"/><i><font color="Green">E58</font></i>: 
( <font color="Maroon">D1</font> <a href="classes1.html#R1" target="_self">is_Tarski-Class_of</a> <font color="Maroon">A</font> &amp; ( for <font color="Olive">D</font> being   <a href="hidden.html#M1">set</a>   st <font color="Olive">D</font> <a href="classes1.html#R1" target="_self">is_Tarski-Class_of</a> <font color="Maroon">A</font> holds <br/><font color="Maroon">D1</font> <a href="tarski.html#R1">c=</a> <font color="Olive">D</font> ) )
 <b>and </b><br/><a NAME="E2:5_1_2"/><i><font color="Green">E59</font></i>: 
( <font color="Maroon">D2</font> <a href="classes1.html#R1" target="_self">is_Tarski-Class_of</a> <font color="Maroon">A</font> &amp; ( for <font color="Olive">D</font> being   <a href="hidden.html#M1">set</a>   st <font color="Olive">D</font> <a href="classes1.html#R1" target="_self">is_Tarski-Class_of</a> <font color="Maroon">A</font> holds <br/><font color="Maroon">D2</font> <a href="tarski.html#R1">c=</a> <font color="Olive">D</font> ) )
 ;<br/>

<b>thus </b><a NAME="E3:5_1_2"/>
( <font color="Maroon">D1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">D2</font> &amp; <font color="Maroon">D2</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">D1</font> )
 <b>by </b><i>, </i>; <i><font color="Red">:: according to </font></i><a class="ref" href="xboole_0.html#D10">XBOOLE_0:def 10</a><br/>


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