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

<b>let </b><font color="Maroon">A</font> be  non <a href="xboole_0.html#V1">empty</a> <a href="finsub_1.html#V4">preBoolean</a>  <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">B</font> be  non <a href="xboole_0.html#V1">empty</a> <a href="finsub_1.html#V4">preBoolean</a>  <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">a</font> be    <a href="subset_1.html#M1">Element</a> of <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">A</font>,<font color="Maroon">B</font></span><a href="zfmisc_1.html#K2">:]</a></span>;<br/><b>let </b><font color="Maroon">b</font> be    <a href="subset_1.html#M1">Element</a> of <span class="p1"><a href="zfmisc_1.html#K2">[:</a><span class="default"><font color="Maroon">A</font>,<font color="Maroon">B</font></span><a href="zfmisc_1.html#K2">:]</a></span>;<br/>







<b>assume </b><a NAME="E1:2"/>
( <font color="Maroon">a</font> <a href="domain_1.html#K2">`1</a>  <a href="tarski.html#R1">c=</a> <font color="Maroon">b</font> <a href="domain_1.html#K2">`1</a>  &amp; <font color="Maroon">a</font> <a href="domain_1.html#K3">`2</a>  <a href="tarski.html#R1">c=</a> <font color="Maroon">b</font> <a href="domain_1.html#K3">`2</a>  &amp; <font color="Maroon">b</font> <a href="domain_1.html#K2">`1</a>  <a href="tarski.html#R1">c=</a> <font color="Maroon">a</font> <a href="domain_1.html#K2">`1</a>  &amp; <font color="Maroon">b</font> <a href="domain_1.html#K3">`2</a>  <a href="tarski.html#R1">c=</a> <font color="Maroon">a</font> <a href="domain_1.html#K3">`2</a>  )
 ; <i><font color="Red">:: according to </font></i><a class="ref" href="normform.html#D1" target="_self">NORMFORM:def 1</a><br/>

<a NAME="E2:2"/><b>then </b>
( <font color="Maroon">a</font> <a href="domain_1.html#K2">`1</a>  <a href="hidden.html#R1">=</a> <font color="Maroon">b</font> <a href="domain_1.html#K2">`1</a>  &amp; <font color="Maroon">a</font> <a href="domain_1.html#K3">`2</a>  <a href="hidden.html#R1">=</a> <font color="Maroon">b</font> <a href="domain_1.html#K3">`2</a>  )
 
<b>by </b><i><a class="ref" href="xboole_0.html#D10">XBOOLE_0:def 10</a></i>;<br/>
<b>hence </b><a NAME="E3:2"/>
<font color="Maroon">a</font> <a href="hidden.html#R1">=</a> <font color="Maroon">b</font>
 <b>by </b><i><a class="ref" href="domain_1.html#T12">DOMAIN_1:12</a></i>;<br/>


</div>
