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<b>let </b><font color="Maroon">V</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">M</font> be  non <a href="struct_0.html#V3">empty</a>  <a href="metric_1.html#L1">MetrStruct</a> ;<br/>



<a NAME="E1:24"/><i><font color="Green">E28</font></i>: 
( <font color="Maroon">V</font> <a href="hidden.html#R2">in</a> <font color="Maroon">M</font> <a href="metric_2.html#K2" target="_self">-neighbour</a>  implies  ex <font color="Olive">x</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">M</font> st <font color="Maroon">V</font> <a href="hidden.html#R1">=</a> <font color="Olive">x</font> <a href="metric_2.html#K1" target="_self">-neighbour</a>  )
 
<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="24_1"><b>proof </b></a><div class="add">

<b>assume </b><a NAME="E1:24_1"/>
<font color="Maroon">V</font> <a href="hidden.html#R2">in</a> <font color="Maroon">M</font> <a href="metric_2.html#K2" target="_self">-neighbour</a> 
 ;<br/>

<a NAME="E2:24_1"/><b>then </b>
<font color="Maroon">V</font> <a href="hidden.html#R2">in</a> <span class="p1">{<span class="default"> <font color="Olive">s</font> where <font color="Olive">s</font> is   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">M</font> :  ex <font color="Olive">x</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">M</font> st <font color="Olive">s</font> <a href="hidden.html#R1">=</a> <font color="Olive">x</font> <a href="metric_2.html#K1" target="_self">-neighbour</a>  </span>}</span> 
 
;<br/>
<a NAME="E3:24_1"/><b>then </b>
 ex <font color="Olive">q</font> being  <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">M</font> st <br/>( <font color="Olive">q</font> <a href="hidden.html#R1">=</a> <font color="Maroon">V</font> &amp;  ex <font color="Olive">x</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">M</font> st <font color="Olive">q</font> <a href="hidden.html#R1">=</a> <font color="Olive">x</font> <a href="metric_2.html#K1" target="_self">-neighbour</a>  )
 
;<br/>
<b>hence </b><a NAME="E4:24_1"/>
 ex <font color="Olive">x</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">M</font> st <font color="Maroon">V</font> <a href="hidden.html#R1">=</a> <font color="Olive">x</font> <a href="metric_2.html#K1" target="_self">-neighbour</a> 
 ;<br/>


</div><b>end;</b></div>
<b>thus </b><a NAME="E2:24"/>
( <font color="Maroon">V</font> <a href="hidden.html#R2">in</a> <font color="Maroon">M</font> <a href="metric_2.html#K2" target="_self">-neighbour</a>  iff  ex <font color="Olive">x</font> being  <a href="struct_0.html#NM1">Element</a> of <font color="Maroon">M</font> st <font color="Maroon">V</font> <a href="hidden.html#R1">=</a> <font color="Olive">x</font> <a href="metric_2.html#K1" target="_self">-neighbour</a>  )
 <b>by </b><i/>;<br/>


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