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<div><span class="kw">theorem </span><a NAME="T13"><span class="comment"><font color="firebrick">:: GATE_1:13</font></span><br/></a><div class="add"> for <font color="Olive" title="b1">a</font>, <font color="Olive" title="b2">b</font> being    <a href="hidden.html#M2" title="HIDDEN:mode.2">set</a>  holds <br/> (  not  <a href="gate_1.html#K7" title="GATE_1:func.7">NOR2</a> (<font color="Olive" title="b1">a</font>,<font color="Olive" title="b2">b</font>) is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  iff ( <font color="Olive" title="b1">a</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  &amp; <font color="Olive" title="b2">b</font> is  <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a>  ) ) ;<br/></div></div>
