def sublist_bigrams(pos_list):
    """ bigrams at sublist level
    >>> sublist_bigrams(['V V', 'R , V O V D N', 'P O V'])
    [('V', 'V'), ('R', ','), (',', 'V'), ('V', 'O'), ('P', 'O'), ('O', 'V')]
    """
    pos_list = [s.split() for s in pos_list]

    bigrams = [
        (a, b)
        for poss in pos_list
        for a, b in zip(poss, poss[1:])
    ]

    return bigrams