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<b>let </b><font color="Maroon">A</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">B</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">C</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">D</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">Z</font> be    <a href="hidden.html#M1">set</a> ;<br/>

<b>assume </b><a NAME="E1:2"/>
( <font color="Maroon">A</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font> &amp; <font color="Maroon">B</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font> &amp; <font color="Maroon">C</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font> )
 ;<br/>

<a NAME="E2:2"/><b>then </b>
<span class="p1">(<span class="default"><font color="Maroon">A</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">B</font></span>)</span> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">C</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font>
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E3:2"/>
(  not <font color="Maroon">D</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font> or <span class="p1">(<span class="default"><span class="p2">(<span class="default"><font color="Maroon">A</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">B</font></span>)</span> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">C</font></span>)</span> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">D</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Z</font> )
 <b>by </b><i><a class="ref" href="xboole_1.html#T8">XBOOLE_1:8</a></i>;<br/>


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