def remove_unused_Y(ar_iteration, dict_Y_predicted, dict_Y_to_remove):
    """Remove unused Y predictions of past AR iterations."""
    list_idx_Y_to_remove = dict_Y_to_remove[ar_iteration]
    if list_idx_Y_to_remove is not None:
        for ldt in list_idx_Y_to_remove: 
            del dict_Y_predicted[ldt]
    return None