import random
import string


def letter(random=random, *args, **kwargs):
    """
    Return a letter!

    >>> mock_random.seed(0)
    >>> letter(random=mock_random)
    'a'
    >>> letter(random=mock_random)
    'b'
    >>> letter(random=mock_random, capitalize=True)
    'C'
    """

    return random.choice(string.ascii_lowercase)