def pairwise(lst):
    """ yield item i and item i+1 in lst. e.g.
        (lst[0], lst[1]), (lst[1], lst[2]), ..., (lst[-1], None)
    """
    if not lst: return
    #yield None, lst[0]
    for i in range(len(lst)-1):
        yield lst[i], lst[i+1]
    yield lst[-1], None