The model supports nested transactions, where the commit of a nested transaction is dependent on the containing parent transaction's commit. Type Transaction defines operations to begin, commit, abort and checkpoint a transaction. A transaction obtains standard read and write locks with pessimistic concurrency control. Binding The model supports run-time binding of methods to objects based on the first argument type. The model supports polymorphism implicit in subtyping, as well as parameterized collection types.
All objects are instances of a type which specifies the interface for accessing the object.
There is only one interface for a type. Copy All denotable objects have an identity. In order to copy objects, copy operations must be defined as part of the object's type. For Collection object types, the copy operation is a shallow copy - returning a new collection with the same elements as the copied collection. For object type Structure, the copy operation is also a shallow copy.