@staticmethod
def commonOffsetsMaxFraction(a, b):
    """
        Compute the fraction of a cluster's sites that are in common with
        another cluster. Do this for two clusters and return the higher
        of the two fractions. E.g., Suppose one cluster has sites 10, 12, 14,
        and 15 and the other cluster has sites 14, 15, ..., 50. Then there
        are two sites in common (14 and 15) and the fractions are 2/4 and 2/37,
        so we return 0.5 (i.e., 2/4) because it is the larger of the two
        fractions. We can use this maximum fraction to tell whether, given a
        pair of clusters, the sites they have in common are a high proportion
        of the total sites for either of the clusters.

        @param a: A C{ReadCluster} instance.
        @param b: A C{ReadCluster} instance.
        @raise ZeroDivisionError: if C{a} and C{b} have no offsets (neither of
            which should be possible in normal operation).
        @return: The C{float} maximum fraction.
        """
    common = set(a.nucleotides) & set(b.nucleotides)
    return len(common) / min(len(a.nucleotides), len(b.nucleotides))
