def specificity(tp=None, tn=None, fp=None, fn=None):
    """Specificity [0, 1]
    
    This function is designed to be used with the confusion performance
    function below. If you only want to specify tp and fn, please use keyword
    arguments when calling.
    
    :param int tp: number of true positives
    :param int tn: number of true negatives
    :param int fp: number of false positives
    :param int fn: number of false negatives
    :rtype: float 
    """
    return tp / float(tp + fn)
