The interface of So-o codes the functions used to define a class and send a message to a class or an instance. The Object Layer codes the containers for a class and an instance and all the functions needed by the Object class. The Object class implements the base messages of the So-o model. A Once class creates only one instance. An Application can be associated to a proxy which will be forwarded all the unknown messages it receives. A Responder automatically sends notification messages to a group of instances.