def get_list_of(key, ch_groups):
    """
    Return a list of values for key in ch_groups[groups]

    key (str): a key of the ch_groups[groups] dict
    ch_groups (dict): a group-name-indexed dict whose values are
        dictionaries of group information (see expand_ch_groups)

    Example: get_list_of('system', ch_groups)
        If ch_groups is the one specified in the json in the expand_ch_groups
        example, this will return the list [ 'geds', 'spms', 'auxs' ]
    """
    values = []
    for ch_info in ch_groups.values():
        if key in ch_info and ch_info[key] not in values: 
            values.append(ch_info[key])
    return values