def rivers_with_station(stations):
    """
    Returns a set of all rivers monitored by 'stations'.

    'stations' is a list of 'MonitoringStation' objects. Documentation for object 'station' can be found
    by importing 'station' from 'floodsystem' and typing 'help(station.MonitoringStation)'

    """

    river_list = []
    for station in stations:
        river_list.append(station.river)

    return set(river_list)