def swap_dict(old_dict):
    """Returns a new dictionary in wich the keys are all the values of the old
    dictionary and the values are lists of keys that had that value.
    
    Example: 
    d = { 'a':['c','v','b'], 's':['c','v','d']} 
    swap_dict(d) -> {'c': ['a', 's'], 'b': ['a'], 'd': ['s'], 'v': ['a', 's']}
    """
    new_dict = {}
    for key in old_dict:
        for value in old_dict[key]:
            new_dict[value] = new_dict.get(value, [])
            new_dict[value].append(key)
    return new_dict