<?xml version="1.0"?>
<div class="add">

<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">P</font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">FT</font>;<br/>



<a NAME="E1:20"/>
( <font color="Maroon">P</font> <a href="subset_1.html#K3">`</a>  is <a href="fin_topo.html#V3">open</a> implies <span class="p1">(<span class="default"><font color="Maroon">P</font> <a href="subset_1.html#K3">`</a> </span>)</span> <a href="subset_1.html#K3">`</a>  is <a href="fin_topo.html#V4">closed</a> )
 
<b>by </b><i><a class="ref" href="fin_topo.html#T30">FIN_TOPO:30</a></i>;<br/>
<b>hence </b><a NAME="E2:20"/>
( <font color="Maroon">P</font> is <a href="fin_topo.html#V4">closed</a> iff <font color="Maroon">P</font> <a href="subset_1.html#K3">`</a>  is <a href="fin_topo.html#V3">open</a> )
 <b>by </b><i><a class="ref" href="fin_topo.html#T31">FIN_TOPO:31</a></i>;<br/>


</div>
