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<pattern id="a" />
<pattern id="b" >
<rect fill="url(#a)"/>
</pattern>
<pattern id="a" xlink:href="#b"/>
<pattern id="b" >
<rect fill="url(#a)"/>
</pattern>
<pattern id="a" xlink:href="#b"/>
<rect fill="url(#a)"/>
</defs>
</svg>