def untag(tagged_sentence):
    """
    Given a tagged sentence, return an untagged version of that
    sentence.  I.e., return a list containing the first element
    of each tuple in C{tagged_sentence}.

    >>> untag([('John', 'NNP'), ('saw', 'VBD'), ('Mary', 'NNP')]
    ['John', 'saw', 'mary']
    """
    return [w for (w, t) in tagged_sentence]