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<span class="kw">let </span><font color="Maroon" title="c1">a</font>, <font color="Maroon" title="c2">b</font> be    <a href="hidden.html#M1" title="HIDDEN:mode.1">set</a> ; <a class="txt" onclick="hs(this)" href="javascript:()"><span class="comment"><font color="Red">::  thesis: </font></span></a><span class="hide"> ( not  <a href="gate_1.html#K5" title="GATE_1:func.5">EQV2</a> <font color="Maroon" title="c1">a</font>,<font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  iff  <a href="gate_1.html#K4" title="GATE_1:func.4">XOR2</a> <font color="Maroon" title="c1">a</font>,<font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  )</span><br/>



<a NAME="E1:21"/>
( not  <a href="gate_1.html#K5" title="GATE_1:func.5">EQV2</a> <font color="Maroon" title="c1">a</font>,<font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  iff ( not <font color="Maroon" title="c1">a</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  iff  not <font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  ) )
 
;<br/>
<span class="kw">hence </span><a NAME="E2:21"/>
( not  <a href="gate_1.html#K5" title="GATE_1:func.5">EQV2</a> <font color="Maroon" title="c1">a</font>,<font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  iff  <a href="gate_1.html#K4" title="GATE_1:func.4">XOR2</a> <font color="Maroon" title="c1">a</font>,<font color="Maroon" title="c2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  )
 ; <a class="txt" onclick="hs(this)" href="javascript:()"><span class="comment"><font color="Red">::  thesis: </font></span></a><span class="hide"> verum</span><br/>


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