Reference

Deploy ddd Classes and Types Call use_ddd at the project outset. Only use once per project.
use_ddd()	Make a Project Independent of the ddd Package
Add Domain Objects
add_entity()	Add an Entity to a Domain
add_value_object()	Add a Value Object to a Domain
add_domain_service()	Add a Service to a Domain
Abstract Base Classes
AbstractDomainService	Abstract Domain Service
AbstractEntity	Abstract Entity Class