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Toward the end of his long life he became a diplomat , and carried out several diplomatic missions on behalf of Pope Callixtus II , Holy Roman Emperor Frederick Barbarossa and Alfonso VII of Castile .
 Pope Callixtus II, Holy Roman Emperor Frederick Barbarossa, Alfonso VII of Castile
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Pope Callixtus II, Holy Roman Emperor Frederick Barbarossa, Alfonso VII of Castile