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<b>let </b><font color="Maroon">G</font> be   <a href="glib_000.html#NM2">_Graph</a>;<br/><b>let </b><font color="Maroon">X1</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">X2</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">Y1</font> be    <a href="hidden.html#M1">set</a> , <font color="Maroon">Y2</font> be    <a href="hidden.html#M1">set</a> ;<br/>



<b>assume </b><a NAME="E1:137"/><i><font color="Green">E28</font></i>: 
( <font color="Maroon">X1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">X2</font> &amp; <font color="Maroon">Y1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">Y2</font> )
 ;<br/>

<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="137_1"><b>now </b></a><div class="add"><b>let </b><font color="Maroon">e</font> be    <a href="hidden.html#M1">set</a> ;<br/><b>assume </b><a NAME="E1:137_1"/>
<font color="Maroon">e</font> <a href="hidden.html#R2">in</a> <font color="Maroon">G</font> <a href="glib_000.html#K24" target="_self">.edgesDBetween</a> <font color="Maroon">X1</font>,<font color="Maroon">Y1</font>
 ;<br/><a NAME="E2:137_1"/><b>then </b>
<font color="Maroon">e</font> <a href="glib_000.html#R4" target="_self">DSJoins</a> <font color="Maroon">X1</font>,<font color="Maroon">Y1</font>,<font color="Maroon">G</font>
 <b>by </b><i/>;<br/><a NAME="E3:137_1"/><b>then </b>
( <font color="Maroon">e</font> <a href="hidden.html#R2">in</a>  <a href="glib_000.html#K7" target="_self">the_Edges_of</a> <font color="Maroon">G</font> &amp; <span class="p1">(<span class="default"><a href="glib_000.html#K10" target="_self">the_Source_of</a> <font color="Maroon">G</font></span>)</span> <a href="funct_1.html#K1">.</a> <font color="Maroon">e</font> <a href="hidden.html#R2">in</a> <font color="Maroon">X1</font> &amp; <span class="p1">(<span class="default"><a href="glib_000.html#K11" target="_self">the_Target_of</a> <font color="Maroon">G</font></span>)</span> <a href="funct_1.html#K1">.</a> <font color="Maroon">e</font> <a href="hidden.html#R2">in</a> <font color="Maroon">Y1</font> )
 <b>by </b><i/>;<br/><a NAME="E4:137_1"/><b>then </b>
<font color="Maroon">e</font> <a href="glib_000.html#R4" target="_self">DSJoins</a> <font color="Maroon">X2</font>,<font color="Maroon">Y2</font>,<font color="Maroon">G</font>
 <b>by </b><i>, </i>;<br/><b>hence </b><a NAME="E5:137_1"/>
<font color="Maroon">e</font> <a href="hidden.html#R2">in</a> <font color="Maroon">G</font> <a href="glib_000.html#K24" target="_self">.edgesDBetween</a> <font color="Maroon">X2</font>,<font color="Maroon">Y2</font>
 <b>by </b><i/>;<br/></div><b>end;</b></div>
<b>hence </b><a NAME="E3:137"/>
<font color="Maroon">G</font> <a href="glib_000.html#K24" target="_self">.edgesDBetween</a> <font color="Maroon">X1</font>,<font color="Maroon">Y1</font> <a href="tarski.html#R1">c=</a> <font color="Maroon">G</font> <a href="glib_000.html#K24" target="_self">.edgesDBetween</a> <font color="Maroon">X2</font>,<font color="Maroon">Y2</font>
 <b>by </b><i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a></i>;<br/>


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