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<b>let </b><font color="Maroon">T</font> be  non <a href="struct_0.html#V3">empty</a> <a href="pre_topc.html#NM1">TopSpace</a>;<br/><b>let </b><font color="Maroon">x</font> be   <a href="pre_topc.html#NM2">Point</a> of <font color="Maroon">T</font>;<br/><b>let </b><font color="Maroon">B</font> be    <a href="hidden.html#M1">set</a> ;<br/>





<a NAME="E1:3"/>
( <font color="Maroon">B</font> <a href="hidden.html#R2">in</a>  <a href="yellow19.html#K1" target="_self">NeighborhoodSystem</a> <font color="Maroon">x</font> iff  ex <font color="Olive">A</font> being   <a href="connsp_2.html#M1">a_neighborhood</a> of <font color="Maroon">x</font> st <font color="Maroon">B</font> <a href="hidden.html#R1">=</a> <font color="Olive">A</font> )
 
;<br/>
<b>hence </b><a NAME="E2:3"/>
( <font color="Maroon">B</font> <a href="hidden.html#R2">in</a>  <a href="yellow19.html#K1" target="_self">NeighborhoodSystem</a> <font color="Maroon">x</font> iff <font color="Maroon">B</font> is    <a href="connsp_2.html#M1">a_neighborhood</a> of <font color="Maroon">x</font> )
 ;<br/>


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