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		<text> 0</text>
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		<text> 5</text>
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		<text> 10</text>
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		<text> 15</text>
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		<text> 20</text>
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		<text> 25</text>
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		<text> 30</text>
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		<text> 1980</text>
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		<text> 1990</text>
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		<text> 2000</text>
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		<text>Number of references</text>
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		<text>Year of publication</text>
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