def mark_time(times, msg=None):
    """
    Time measurement utility.

    Measures times of execution between subsequent calls using
    time.clock(). The time is printed if the msg argument is not None.

    Examples
    --------

    >>> times = []
    >>> mark_time(times)
    ... do something
    >>> mark_time(times, 'elapsed')
    elapsed 0.1
    ... do something else
    >>> mark_time(times, 'elapsed again')
    elapsed again 0.05
    >>> times
    [0.10000000000000001, 0.050000000000000003]
    """
    tt = time.clock()
    times.append(tt)
    if (msg is not None) and (len(times) > 1):
        print msg, times[-1] - times[-2]