code: @Pure
  	public Iterable<BusHub> busHubs() {
  		final MultiCollection<BusHub> col = new MultiCollection<>();
  		col.addCollection(this.validBusHubs);
  		col.addCollection(this.invalidBusHubs);
  		return Collections.unmodifiableCollection(col);
  	}
docs: Replies the list of the bus hubs of the bus network.
  
  @return a list of bus hubs