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<b>let </b><font color="Maroon">c<sub>1</sub></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">c<sub>2</sub></font> be    <a href="pre_topc.html#M1">SubSpace</a> of <font color="Maroon">c<sub>1</sub></font>;<br/><b>let </b><font color="Maroon">c<sub>3</sub></font> be   <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">c<sub>1</sub></font>;<br/>





<b>assume </b><a NAME="E1:38"/><i><font color="Green">E17</font></i>: 
<font color="Maroon">c<sub>3</sub></font> <a href="hidden.html#R1">=</a> the <a href="struct_0.html#U1">carrier</a> of <font color="Maroon">c<sub>2</sub></font>
 ;<br/>

<b>hence </b><a NAME="E2:38"/>
( <font color="Maroon">c<sub>2</sub></font> is <a href="tex_3.html#V3" target="_self">boundary</a> implies <font color="Maroon">c<sub>3</sub></font> is <a href="tops_1.html#V2">boundary</a> )
 <b>by </b><i><a class="ref" href="tex_3.html#D3" target="_self">Def3</a></i>;<br/>

<b>assume </b><a NAME="E3:38"/>
<font color="Maroon">c<sub>3</sub></font> is <a href="tops_1.html#V2">boundary</a>
 ;<br/>

<a NAME="E4:38"/><b>then </b>
for <font color="Olive">b<sub>1</sub></font> being  <a href="struct_0.html#NM2">Subset</a> of <font color="Maroon">c<sub>1</sub></font> st <font color="Olive">b<sub>1</sub></font> <a href="hidden.html#R1">=</a> the <a href="struct_0.html#U1">carrier</a> of <font color="Maroon">c<sub>2</sub></font> holds <br/><font color="Olive">b<sub>1</sub></font> is <a href="tops_1.html#V2">boundary</a>
 
<b>by </b><i><a class="txt" href="tex_3.html#E1:38"><i><font color="Green">E17</font></i></a></i>;<br/>
<b>hence </b><a NAME="E5:38"/>
<font color="Maroon">c<sub>2</sub></font> is <a href="tex_3.html#V3" target="_self">boundary</a>
 <b>by </b><i><a class="ref" href="tex_3.html#D3" target="_self">Def3</a></i>;<br/>


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