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<b>let </b><font color="Maroon">FT</font> be  non <a href="struct_0.html#V3">empty</a>  <a href="orders_2.html#L1">RelStr</a> ;<br/><b>let </b><font color="Maroon">x</font> be   <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font>;<br/><b>let </b><font color="Maroon">A</font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">FT</font>;<br/>





<a NAME="E1:18"/><i><font color="Green">E19</font></i>: 
( <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> <a href="fin_topo.html#K12">^f</a>  implies  ex <font color="Olive">y</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font> st <br/>(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Olive">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Olive">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  ) )
 
<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="18_1"><b>proof </b></a><div class="add">

<b>assume </b><a NAME="E1:18_1"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> <a href="fin_topo.html#K12">^f</a> 
 ;<br/>

<b>then consider </b><font color="Maroon">y</font> being   <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font><b> such that </b><br/><a NAME="E3:18_1"/><i><font color="Green">E21</font></i>: 
( <font color="Maroon">y</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> &amp; <font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="fin_topo.html#K1">U_FT</a> <font color="Maroon">y</font> )
 <b>by </b><i><a class="ref" href="fin_topo.html#T16">FIN_TOPO:16</a></i>;<br/>
<a NAME="E4:18_1"/><i><font color="Green">E23</font></i>: 
 <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Maroon">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a> 
 
<b>by </b><i>, </i>;<br/>
<a NAME="E5:18_1"/>
 <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Maroon">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a> 
 
<b>by </b><i>, </i>;<br/>
<b>hence </b><a NAME="E6:18_1"/>
 ex <font color="Olive">y</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font> st <br/>(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Olive">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Olive">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  )
 <b>by </b><i/>;<br/>


</div><b>end;</b></div>
<a NAME="E2:18"/>
(  ex <font color="Olive">y</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font> st <br/>(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Olive">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Olive">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  ) implies <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> <a href="fin_topo.html#K12">^f</a>  )
 
<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="18_2"><b>proof </b></a><div class="add">

<b>assume </b><a NAME="E1:18_2"/>
 ex <font color="Olive">y</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font> st <br/>(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Olive">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Olive">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  )
 ;<br/>

<b>then consider </b><font color="Maroon">y</font> being   <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font><b> such that </b><br/><a NAME="E3:18_2"/><i><font color="Green">E33</font></i>: 
(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Maroon">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Maroon">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  )
 ;<br/>
<a NAME="E4:18_2"/><i><font color="Green">E35</font></i>: 
<font color="Maroon">y</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font>
 
<b>by </b><i>, </i>;<br/>
<a NAME="E5:18_2"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="fin_topo.html#K1">U_FT</a> <font color="Maroon">y</font>
 
<b>by </b><i>, </i>;<br/>
<b>hence </b><a NAME="E6:18_2"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> <a href="fin_topo.html#K12">^f</a> 
 <b>by </b><i>, <a class="ref" href="fin_topo.html#T16">FIN_TOPO:16</a></i>;<br/>


</div><b>end;</b></div>
<b>hence </b><a NAME="E3:18"/>
( <font color="Maroon">x</font> <a href="hidden.html#R2">in</a> <font color="Maroon">A</font> <a href="fin_topo.html#K12">^f</a>  iff  ex <font color="Olive">y</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">FT</font> st <br/>(  <a href="fintopo2.html#K4" target="_self">P_A</a> <font color="Olive">y</font>,<font color="Maroon">A</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  &amp;  <a href="fintopo2.html#K3" target="_self">P_0</a> <font color="Olive">y</font>,<font color="Maroon">x</font> <a href="hidden.html#R1">=</a>  <a href="margrel1.html#K8">TRUE</a>  ) )
 <b>by </b><i/>;<br/>


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