def join_ensembles(ensemble_list):
    """Join several ensembles using a set theory union.

    Parameters
    ----------
    ensemble_list : list of :class:`.Ensemble`
        list of ensembles to join

    Returns
    -------
    :class:`.Ensemble`
        union of all given ensembles
    """
    ensemble = None
    for ens in ensemble_list:
        if ensemble is None:
            ensemble = ens
        else:
            ensemble = ensemble | ens
    return ensemble