def stringToWordList(s):
    """Returns t.split() where t is obtained from s.lower() by replacing
    every non-letter in this string with a blank. Basically, it returns
    a list of the 'words' (sequences of letters) in s.

    PreC: s is a string.
    """
    t = ''
    for c in s.lower():
        if c in 'abcdefghijklmnopqrstuvwxyz':
            t = t + c
        else:
            t = t + ' '
    return t.split()