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<b>let </b><font color="Maroon">x</font> be    <a href="hidden.html#M1">set</a> ;<br/>

<a NAME="E1:10"/>
(  <a href="trees_4.html#K1" target="_self">root-tree</a> <font color="Maroon">x</font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><a href="trees_1.html#K2">elementary_tree</a> 0</span>)</span> <a href="funcop_1.html#K2">--&gt;</a> <font color="Maroon">x</font> &amp;  <a href="xboole_0.html#K1">{}</a>  <a href="hidden.html#R2">in</a>  <a href="trees_1.html#K2">elementary_tree</a> 0 )
 
<b>by </b><i><a class="ref" href="tarski.html#D1">TARSKI:def 1</a>, <a class="ref" href="trees_1.html#T56">TREES_1:56</a></i>;<br/>
<b>hence </b><a NAME="E2:10"/>
(  <a href="relat_1.html#K1">dom</a> <span class="p1">(<span class="default"><a href="trees_4.html#K1" target="_self">root-tree</a> <font color="Maroon">x</font></span>)</span> <a href="hidden.html#R1">=</a>  <a href="trees_1.html#K2">elementary_tree</a> 0 &amp; <span class="p1">(<span class="default"><a href="trees_4.html#K1" target="_self">root-tree</a> <font color="Maroon">x</font></span>)</span> <a href="funct_1.html#K1">.</a> <a href="xboole_0.html#K1">{}</a>  <a href="hidden.html#R1">=</a> <font color="Maroon">x</font> )
 <b>by </b><i><a class="ref" href="funcop_1.html#T13">FUNCOP_1:13</a>, <a class="ref" href="funcop_1.html#T19">FUNCOP_1:19</a></i>;<br/>


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