>>> from letters import vowel_occurrences

>>> d = vowel_occurrences('Genius is 1% inspiration and 99% perspiration. Thomas Edison.')
>>> if d != {'a': 4, 'e': 3, 'i': 8, 'o': 4, 'u': 1}:
...    print(d)

>>> d = vowel_occurrences('Three can keep a secret, if two of them are dead. Benjamin Franklin')
>>> if d != {'a': 6, 'e': 10, 'i': 3, 'o': 2, 'u': 0}:
...    print(d)

>>> d = vowel_occurrences("I'm gonna make him an offer he can't refuse. Vito Corleone.")
>>> if d != {'a': 4, 'e': 7, 'i': 3, 'o': 5, 'u': 1}:
...    print(d)

>>> d = vowel_occurrences('U.S. Route 61 (US 61), 1400 miles of blues')
>>> if d != {'a': 0, 'e': 3, 'i': 1, 'o': 2, 'u': 4}:
...    print(d)