def get_past_cone(past_states, location, neighbors_fn):
    """
    Get the past light cone given past states at the provided location.
    The neighbors_fn should take a state, location and radius and return the
    list of neighbors.
    """
    t = len(past_states)
    return tuple(neighbors_fn(state, location, t - i - 1)
                     for i, state in enumerate(past_states))