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

<a NAME="E1:41"/>
 <a href="trees_4.html#K1">root-tree</a> <font color="Maroon">x</font> <a href="hidden.html#R1">=</a> <span class="p1"><a href="tarski.html#K1">{</a><span class="default"><span class="p2"><a href="tarski.html#K4">[</a><span class="default"><a href="xboole_0.html#K1">{}</a> ,<font color="Maroon">x</font></span><a href="tarski.html#K4">]</a></span></span><a href="tarski.html#K1">}</a></span>
 
<b>by </b><i><a class="ref" href="trees_4.html#T6">TREES_4:6</a></i>;<br/>
<b>hence </b><a NAME="E2:41"/>
 <a href="card_1.html#K4">card</a> <span class="p1">(<span class="default"><a href="trees_4.html#K1">root-tree</a> <font color="Maroon">x</font></span>)</span> <a href="hidden.html#R1">=</a> 1
 <b>by </b><i><a class="ref" href="card_1.html#T79">CARD_1:79</a></i>;<br/>


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