'Scores bigrams using Fisher\'s Exact Test (Pedersen 1996).  Less
sensitive to small counts than PMI or Chi Sq, but also more expensive
to compute. Requires scipy.'

@classmethod
def fisher(cls, *marginals):

    (n_ii, n_io, n_oi, n_oo) = cls._contingency(*marginals)
    (odds, pvalue) = fisher_exact([[n_ii, n_io], [n_oi, n_oo]], alternative='less')
    return pvalue
