Subroutine for ceafe. Computes the mention F measure between gold and
        predicted mentions in a cluster.

def phi4(gold_clustering, predicted_clustering):
        """
        Subroutine for ceafe. Computes the mention F measure between gold and
        predicted mentions in a cluster.
        """
        return 2 * len([mention for mention in gold_clustering if mention in predicted_clustering]) \
               / float(len(gold_clustering) + len(predicted_clustering))