Should be able to delete the Car...
!|using naked objects viewer|
|on object|perform|using member|that it|with arguments|
|joeBloggsCustomer|check action|delete vehicle|is valid for|joeFordFocusCar|

... and indeed we can
!|using naked objects viewer|
|on object|perform|using member|that it|with arguments|
|joeBloggsCustomer|check collection|vehicles|contains|joeFordFocusCar|
||invoke action|delete vehicle|.|joeFordFocusCar|
||check collection|vehicles|does not contain|joeFordFocusCar|
