def append_dempster_attr(ds_list, dempster_label='belief'):
    """
    Helper functiont to append the dempster output type label
    to existing dataset. Just an xarray update function.

    Parameters
    ----------
    ds_list: list
        A list of xarray datasets.

    Returns
    ----------
    out_list : list of xarray datasets with appended attributes.
    """
    
    # check if list
    if not isinstance(ds_list, list):
        ds_list = [ds_list]
    
    # loop xr datasets and append dempster label
    out_list = []
    for ds in ds_list:
        ds.attrs.update({'dempster_type': dempster_label})
        out_list.append(ds)
        
    # return
    return out_list