Consider modelling an application involving airline flights between airports. Each airport has a name $($e$.$g$.,$London Heathrow$)$ and a unique code $($e$.$g$.,$LHR$).$ Each flight has a unique flight number and is between two airports, a source and a desti$-$ nation. $($You can assume that we are not modelling the reality that the same flight number can be used on different days of the week; we are only interested in which flight numbers connect which airports.$)$