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In " Blue Tigers " , the narrator gets hold of a group of mysterious blue stones whose number continuously multiplies and divides when one is not looking ( retaking the themes of his previous stories " The Zahir " , " The Disk " , and " The Book of Sand " : a direct confrontation with the inconceivable , in the form of an impossible object ) . "
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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