def combine_predictions_list(predictions_list, index_list=None):
    """Combine predictions in predictions_list[index_list].

    By taking the mean of their get_combineable_predictions views.

    E.g. for regression it is the actual
    predictions, and for classification it is the probability array (which
    should be calibrated if we want the best performance). Called both for
    combining one submission on cv folds (a single model that is trained on
    different folds) and several models on a single fold.
    Called by
    _get_bagging_score : which combines bags of the same model, trained on
        different folds, on the heldout test set
    _get_cv_bagging_score : which combines cv-bags of the same model, trained
        on different folds, on the training set
    get_next_best_single_fold : which does one step of the greedy forward
        selection (of different models) on a single fold
    _get_combined_predictions_single_fold : which does the full loop of greedy
        forward selection (of different models), until improvement, on a single
        fold
    _get_combined_test_predictions_single_fold : which computes the combination
        (constructed on the cv valid set) on the holdout test set, on a single
        fold
    _get_combined_test_predictions : which combines the foldwise combined
        and foldwise best test predictions into a single megacombination

    Parameters
    ----------
    predictions_list : list of instances of Predictions
        Each element of the list is an instance of Predictions of a given model
        on the same data points.
    index_list : None | list of integers
        The subset of predictions to be combined. If None, the full set is
        combined.

    Returns
    -------
    combined_predictions : instance of Predictions
        A predictions instance containing the combined (averaged) predictions.
    """
    Predictions = type(predictions_list[0])
    combined_predictions = Predictions.combine(predictions_list, index_list)
    return combined_predictions