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time-keeping × 50calendar × 15
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astronomy × 4
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india × 2
social-history × 2
mesopotamia × 2
military × 1
middle-ages × 1
europe × 1
japan × 1
england × 1
germany × 1
china × 1
united-kingdom × 1
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science × 1
spain × 1
hitler × 1
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north-america × 1