def create_dico(item_list):
    """
    Create a dictionary of items from a list of list of items.
    字典 字符:出现的频率
    """
    assert type(item_list) is list
    dico = {}
    for items in item_list:
        for item in items:
            if item not in dico:
                dico[item] = 1
            else:
                dico[item] += 1
    return dico
