using 'only' to extend a method when there is no prior definition

In section Overriding methods it is stated as follows: It is not an error if a method definition contains an only qualifier and there is no existing method to be overridden.

We want to use explicit overriding methods in order to protect the users from typos. It is obvious that a user that explicitly used 'only' want to override an existing method and therefore an error must be issued if the method was not previously declared (The previous declaration can be with or without implementation).