'Construct a BigramCollocationFinder for all bigrams in the given
sequence.  When window_size > 2, count non-contiguous bigrams, in the
style of Church and Hanks\'s (1990) association ratio.'

@classmethod
def from_words(cls, words, window_size=2):

    wfd = FreqDist()
    bfd = FreqDist()
    if (window_size < 2):
        raise ValueError('Specify    window_size    at    least    2')
    for window in ngrams(words, window_size, pad_right=True):
        w1 = window[0]
        if (w1 is None):
            continue
        wfd[w1] += 1
        for w2 in window[1:]:
            if (w2 is not None):
                bfd[(w1, w2)] += 1
    return cls(wfd, bfd, window_size=window_size)
