def calculate_term_frequencies(tokens):
    """Given a series of `tokens`, produces a sorted list of tuples in the
    format of (term frequency, token).
    """
    frequency_dict = {}
    for token in tokens:
        frequency_dict.setdefault(token, 0)
        frequency_dict[token] += 1
    tf = []
    for token, count in frequency_dict.items():
        tf.append((count, token))
    return sorted(tf, reverse=True)
