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Single-issue politics may express itself through the formation of a single-issue party , an approach that tends to be more successful in parliamentary systems based on proportional representation than in rigid two-party systems ( like that of the United States ) .
 location: United States
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 location: United States