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<b>let </b><font color="Maroon">x</font> be   <a href="ordinal1.html#NM2">Ordinal</a>, <font color="Maroon">y</font> be   <a href="ordinal1.html#NM2">Ordinal</a>;<br/>

<b>assume </b><a NAME="E1:9"/><i><font color="Green">E13</font></i>: 
<font color="Maroon">x</font> <a href="ordinal2.html#K11">*^</a> <font color="Maroon">y</font> <a href="hidden.html#R2">in</a>  <a href="ordinal1.html#K5">omega</a> 
 ; <i><font color="Red">:: according to </font></i><a class="ref" href="ordinal1.html#D13">ORDINAL1:def 13</a><br/>

<b>assume </b><a NAME="E2:9"/>
not <font color="Maroon">x</font> <a href="ordinal2.html#K11">*^</a> <font color="Maroon">y</font> is <a href="xboole_0.html#V1">empty</a>
 ;<br/>

<a NAME="E3:9"/><b>then </b>
( <font color="Maroon">x</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a>  <a href="xboole_0.html#K1">{}</a>  &amp; <font color="Maroon">y</font> <a href="hidden.html#NR2" target="_self">&lt;&gt;</a>  <a href="xboole_0.html#K1">{}</a>  )
 
<b>by </b><i><a class="ref" href="ordinal2.html#T52">ORDINAL2:52</a>, <a class="ref" href="ordinal2.html#T55">ORDINAL2:55</a></i>;<br/>
<a NAME="E4:9"/><b>then </b>
( <font color="Maroon">x</font> <a href="ordinal1.html#R1">c=</a> <font color="Maroon">x</font> <a href="ordinal2.html#K11">*^</a> <font color="Maroon">y</font> &amp; <font color="Maroon">y</font> <a href="ordinal1.html#R1">c=</a> <font color="Maroon">x</font> <a href="ordinal2.html#K11">*^</a> <font color="Maroon">y</font> )
 
<b>by </b><i><a class="ref" href="ordinal3.html#T39">ORDINAL3:39</a></i>;<br/>
<b>hence </b><a NAME="E5:9"/>
( <font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="ordinal1.html#K5">omega</a>  &amp; <font color="Maroon">y</font> <a href="hidden.html#R2">in</a>  <a href="ordinal1.html#K5">omega</a>  )
 <b>by </b><i>, <a class="ref" href="ordinal1.html#T22">ORDINAL1:22</a></i>;<br/>


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