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The cousin of his grandfather was Gustav Mahler and his son-in-law was the well known Czech novelist Karel Poláček .
 Gustav Mahler, Czech, Karel Poláček
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Gustav Mahler, Czech, Karel Poláček