def _test_dist(id1, id2):
    """
    Return distance between identifiers id1 and id2.
    The identifiers are of the form 'time=some number'.
    """
    # extract the numbers using regex:
    #t1 = re.search(r"time=(.*)", id1).group(1)
    #t2 = re.search(r"time=(.*)", id2).group(1)
    t1 = id1[5:];  t2 = id2[5:]
    d = abs(float(t1) - float(t2))
    return d