Functionally, the station authority files support several lookup options: look up a location code; find the set of stations in a location; look up a station number; look up an alphabetic station code.
Hence, we define a base class
BaseStationSet that describes the interface for these operations.
Derived classes represent one of the station authority
For the details of this class, see
Section 8, “
class BaseStationSet: Base class for
station authority objects”.