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<b>let </b><font color="Maroon">C</font> be   <a href="ordinal1.html#NM2">Ordinal</a>;<br/><b>let </b><font color="Maroon">A</font> be   <a href="ordinal1.html#NM2">Ordinal</a>;<br/><b>let </b><font color="Maroon">B</font> be   <a href="ordinal1.html#NM2">Ordinal</a>;<br/>





<b>assume </b><a NAME="E1:64"/><i><font color="Green">E19</font></i>: 
<font color="Maroon">C</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a>  <a href="xboole_0.html#K1">{}</a> 
 ;<br/>

<a NAME="E2:64"/><i><font color="Green">E21</font></i>: 
( ( <font color="Maroon">B</font> <a href="ordinal1.html#R1">c=</a> <font color="Maroon">A</font> implies <font color="Maroon">A</font> <a href="hidden.html#R1">=</a> <font color="Maroon">B</font> <a href="ordinal2.html#K10">+^</a> <span class="p1">(<span class="default"><font color="Maroon">A</font> <a href="ordinal3.html#K5" target="_self">-^</a> <font color="Maroon">B</font></span>)</span> ) &amp; ( not <font color="Maroon">B</font> <a href="ordinal1.html#R1">c=</a> <font color="Maroon">A</font> implies <font color="Maroon">A</font> <a href="ordinal3.html#K5" target="_self">-^</a> <font color="Maroon">B</font> <a href="hidden.html#R1">=</a>  <a href="xboole_0.html#K1">{}</a>  ) )
 
<b>by </b><i/>;<br/>
<a NAME="E3:64"/>
<span class="p1">(<span class="default"><font color="Maroon">B</font> <a href="ordinal2.html#K10">+^</a> <font color="Maroon">C</font></span>)</span> <a href="ordinal3.html#K5" target="_self">-^</a> <font color="Maroon">B</font> <a href="hidden.html#R1">=</a> <font color="Maroon">C</font>
 
<b>by </b><i/>;<br/>
<b>hence </b><a NAME="E4:64"/>
(  not <font color="Maroon">A</font> <a href="hidden.html#R2">in</a> <font color="Maroon">B</font> <a href="ordinal2.html#K10">+^</a> <font color="Maroon">C</font> or <font color="Maroon">A</font> <a href="ordinal3.html#K5" target="_self">-^</a> <font color="Maroon">B</font> <a href="hidden.html#R2">in</a> <font color="Maroon">C</font> )
 <b>by </b><i>, , , </i>;<br/>


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