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In machining , boring is the process of enlarging a hole that has already been drilled ( or cast ) , by means of a single-point cutting tool ( or of a boring head containing several such tools ) , for example as in boring a gun barrel or an engine cylinder .
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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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