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Over two distinct datasets , we find that indexing according to simple character bigrams produces a retrieval accuracy superior to any of the tested word N-gram models .
 character bigrams, indexing; character bigrams, word N-gram models; retrieval accuracy, character bigrams; retrieval accuracy, word N-gram models
Please list all entity pairs containing a certain relationship in the given options.Output format is "word1, word2; word3, word4". 
Option: conjunction, feature of, hyponym of, used for, part of, compare, evaluate for 
Text: {0} 
Answer:
 character bigrams, indexing; character bigrams, word N-gram models; retrieval accuracy, character bigrams; retrieval accuracy, word N-gram models