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

</defs>

</svg>