has_add_on
Use it like this:
- [ #has/_add_on :: Offer ] or
- [ has_add_on :: Offer ]
An additional offer that can only be obtained in combination with the first base offer (e.g. supplements and extensions that are available for a surcharge).
Relation describes that: [ #has_/domain :: Offer ] ( #has_/name :: has_add_on ) ( #has_/range :: Offer )