[foreign_key_annotations] EXTEND field : foreign_key_addition | value_help_addition;
Component extensions can be used:
A component extension is introduced with the EXTEND keyword, followed by the name of the component, and a foreign key or value help addition, or both.
If a structure is included, you can remove existing value helps or foreign keys using the REMOVE VALUE HELP or REMOVE FOREIGN KEY keywords.
The zip_code and street components of the address include structure are extended:
... office_address : INCLUDE address EXTEND zip_code: WITH FOREIGN KEY subsideries WHERE plz = employee.zip_code EXTEND street: REMOVE VALUE HELP; ...