Make FrontAxle for vehicles optional to support trailers
The newly introduced example to show the trailer usage contains a trailer definition in the VehicleCatalog which contains only a rear axle. Currently according to the schema / UML model all vehicles also must have a front axle. Since trailers with only one axle are quite common the proposal is to make the frontAxle attribute of the Axles
class optional. Then only a rear axle is mandatory.