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<b>let </b><font color="Maroon" title="c1">I</font> be    <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ;<br/><b>let </b><font color="Maroon" title="c2">X</font>, <font color="Maroon" title="c3">Y</font> be    <a href="pboole.html#M1" title="PBOOLE:mode.1">ManySortedSet</a> of <font color="Maroon" title="c1">I</font>;<br/>





<b>assume </b><b>that </b><br/><a NAME="E1:137"/><i><font color="Green" title="E74">A1</font></i>: 
<font color="Maroon" title="c2">X</font> is <a href="relat_1.html#V2" title="RELAT_1:attr.2">non-empty</a>
 <b>and </b><br/><a NAME="E2:137"/><i><font color="Green" title="E75">A2</font></i>: 
<font color="Maroon" title="c2">X</font> <a href="pboole.html#R2" title="PBOOLE:pred.2">c=</a> <font color="Maroon" title="c3">Y</font>
 ;<br/>

<b>let </b><font color="Maroon" title="c4">i</font> be    <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="pboole.html#D16" target="_self" title="PBOOLE:def.16">PBOOLE:def 16</a><br/>

<b>assume </b><a NAME="E3:137"/><i><font color="Green" title="E76">A3</font></i>: 
<font color="Maroon" title="c4">i</font> <a href="hidden.html#R2" title="HIDDEN:pred.2">in</a> <font color="Maroon" title="c1">I</font>
 ;<br/>

<a NAME="E4:137"/><b>then </b><i><font color="Green" title="E77">A4</font></i>: 
<font color="Maroon" title="c2">X</font> <a href="funct_1.html#K1" title="FUNCT_1:func.1">.</a> <font color="Maroon" title="c4">i</font> <a href="tarski.html#R1" title="TARSKI:pred.1">c=</a> <font color="Maroon" title="c3">Y</font> <a href="funct_1.html#K1" title="FUNCT_1:func.1">.</a> <font color="Maroon" title="c4">i</font>
 
<b>by </b><i><a class="txt" href="pboole.html#E2:137"><i><font color="Green" title="E75">A2</font></i></a>, <a class="ref" href="pboole.html#D5" target="_self" title="PBOOLE:def.5">Def5</a></i>;<br/>
<a NAME="E5:137"/>
not <font color="Maroon" title="c2">X</font> <a href="funct_1.html#K1" title="FUNCT_1:func.1">.</a> <font color="Maroon" title="c4">i</font> is <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>
 
<b>by </b><i><a class="txt" href="pboole.html#E1:137"><i><font color="Green" title="E74">A1</font></i></a>, <a class="txt" href="pboole.html#E3:137"><i><font color="Green" title="E76">A3</font></i></a>, <a class="ref" href="pboole.html#D16" target="_self" title="PBOOLE:def.16">Def16</a></i>;<br/>
<b>hence </b><a NAME="E6:137"/>
not <font color="Maroon" title="c3">Y</font> <a href="funct_1.html#K1" title="FUNCT_1:func.1">.</a> <font color="Maroon" title="c4">i</font> is <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>
 <b>by </b><i><a class="txt" href="pboole.html#E4:137"><i><font color="Green" title="E77">A4</font></i></a>, <a class="ref" href="xboole_1.html#T3" title="XBOOLE_1:th.3">XBOOLE_1:3</a></i>;<br/>


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