the main feature I miss is to be able to declare abstract services (that should never be runned but could be used to group common properties of services for convenience).

I understand that multiple -f options fits almost the bill, but it doesn't always work. For example, I rather often change the name of the abstract service into something that is more meaningful in the context of the project at hand. This is something that the overriding that occurs with multiple -f options does not support. I would not mind loosing the exact extends though, as long as their is some other way to "instantiate" an abstract service in a file.

Provides a template for future specific classes

The concrete nouns include people, plants, animals, materials and objects while the abstract nouns refer to concepts such as qualities, actions, and processes.

There is a base class, AbstractNode, that defines the contract and offers some common functionality, like parent and getDepth().

The following roles are used to support the WAI-ARIA role taxonomy for the purpose of defining general role concepts. Abstract roles are used for the ontology. Authors MUST NOT use abstract roles in content.

Abstract roles are the foundation upon which all other WAI-ARIA roles are built.

Abstract roles are provided to help with the following: Organize the role taxonomy and provide roles with a meaning in the context of known concepts.Streamline the addition of roles that include necessary features.