<?xml version="1.0"?>
<div class="add">

<b>let </b><font color="Maroon">P</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">X1</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">Y1</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">Q</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">X2</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">Y2</font> be    <a href="hidden.html#M1">set</a> ;<br/>











<a NAME="E1:1"/>
( <font color="Maroon">X1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font> &amp; <font color="Maroon">Y2</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font> &amp; <font color="Maroon">X2</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font> &amp; <font color="Maroon">Y1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font> )
 
<b>by </b><i><a class="ref" href="xboole_1.html#T7">XBOOLE_1:7</a></i>;<br/>
<a NAME="E2:1"/><b>then </b>
( <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X1</font>,<font color="Maroon">Y1</font></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span> &amp; <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X2</font>,<font color="Maroon">Y2</font></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span> &amp; <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="xboole_0.html#K2">\/</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="hidden.html#R1">=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span> )
 
<b>by </b><i><a class="ref" href="zfmisc_1.html#T119">ZFMISC_1:119</a></i>;<br/>
<a NAME="E3:1"/><b>then </b><i><font color="Green">E26</font></i>: 
<span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X1</font>,<font color="Maroon">Y1</font></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="xboole_0.html#K2">\/</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X2</font>,<font color="Maroon">Y2</font></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span>
 
<b>by </b><i><a class="ref" href="xboole_1.html#T13">XBOOLE_1:13</a></i>;<br/>
<b>assume </b><a NAME="E4:1"/>
( <font color="Maroon">P</font> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X1</font>,<font color="Maroon">Y1</font></span><a href="zfmisc_1.html#K2">:]</a></span> &amp; <font color="Maroon">Q</font> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X2</font>,<font color="Maroon">Y2</font></span><a href="zfmisc_1.html#K2">:]</a></span> )
 ;<br/>

<a NAME="E5:1"/><b>then </b>
<font color="Maroon">P</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Q</font> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X1</font>,<font color="Maroon">Y1</font></span><a href="zfmisc_1.html#K2">:]</a></span> <a href="xboole_0.html#K2">\/</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">X2</font>,<font color="Maroon">Y2</font></span><a href="zfmisc_1.html#K2">:]</a></span>
 
<b>by </b><i><a class="ref" href="xboole_1.html#T13">XBOOLE_1:13</a></i>;<br/>
<b>hence </b><a NAME="E6:1"/>
<font color="Maroon">P</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Q</font> <a href="tarski.html#R1">c=</a> <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><span class="p2">(<span class="default"><font color="Maroon">X1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">X2</font></span>)</span>,<span class="p2">(<span class="default"><font color="Maroon">Y1</font> <a href="xboole_0.html#K2">\/</a> <font color="Maroon">Y2</font></span>)</span></span><a href="zfmisc_1.html#K2">:]</a></span>
 <b>by </b><i>, <a class="ref" href="xboole_1.html#T1">XBOOLE_1:1</a></i>;<br/>


</div>
