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The historian Warren Treadgold dates the theme 's creation to c. 809 , during the anti-Slavic campaigns of Emperor Nikephoros I ( r. 802 – 811 ) which extended Byzantine rule to the city 's hinterland .
 Warren Treadgold, Nikephoros I, Byzantine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Warren Treadgold, Nikephoros I, Byzantine