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<mml:mi mathvariant="normal">of</mml:mi>
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<mml:mi mathvariant="normal">occurrence</mml:mi>
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<mml:mi mathvariant="normal">X</mml:mi>
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<mml:mi mathvariant="normal">Y</mml:mi>
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<mml:mi mathvariant="normal">for</mml:mi>
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<mml:mi mathvariant="normal">a</mml:mi>
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<mml:mi mathvariant="normal">particular</mml:mi>
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<mml:mi mathvariant="normal">spacer</mml:mi>
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<mml:mi mathvariant="normal">length</mml:mi>
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<mml:mfenced close=")" open="("> <mml:mi mathvariant="normal">N</mml:mi>
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