1: merge ('w', 'e')          (pair occurred 17 times)
 2: merge ('l', 'o')          (pair occurred 13 times)
 3: merge ('r', '</w>')       (pair occurred 12 times)
 4: merge ('s', 't')          (pair occurred 11 times)
 5: merge ('st', '</w>')      (pair occurred 11 times)
 6: merge ('we', 'st</w>')    (pair occurred 9 times)
 7: merge ('n', 'e')          (pair occurred 9 times)
 8: merge ('w', 'i')          (pair occurred 9 times)
 9: merge ('wi', 'd')         (pair occurred 9 times)
10: merge ('wid', 'e')        (pair occurred 9 times)
11: merge ('we', 'r</w>')     (pair occurred 8 times)
12: merge ('lo', 'west</w>')  (pair occurred 6 times)
13: merge ('ne', 'wer</w>')   (pair occurred 6 times)
14: merge ('lo', 'w')         (pair occurred 5 times)
15: merge ('low', '</w>')     (pair occurred 5 times)
