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Composers such as Béla Bartók and , later , Lou Harrison and Karlheinz Stockhausen became proponents of the tone cluster , which feature in the work of many 20th - and 21st - century classical composers .
 person: Béla Bartók; person: Lou Harrison; person: Karlheinz Stockhausen
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Answer:
 person: Béla Bartók; person: Lou Harrison; person: Karlheinz Stockhausen