5.4. Supertyping

Unlike most other object oriented languages, Sather also allows the programmer to introduce types above an existing class. A supertyping clause ('>' followed by type specifiers) adds to the type graph an edge from the type being defined to each type in the supertyping clause. These type specifiers may not be type parameters (though they may include type parameters as components) or external types. There must be no cycle of abstract classes such that each class appears in the supertype list of the next, ignoring the values of any type parameters but not their number. A supertyping clause may not refer to SAME.

If both subtyping and supertyping clauses are present, then each type in the supertyping list must be a subtype of each type in the subtyping list using only edges introduced by subtyping clauses. This ensures that the subtype relationship can be tested by examining only definitions reachable from the two types in question, and that errors of supertyping are localized.You define supertypes of already existing types. The supertype can only contain routines that are found in the subtype i.e. it cannot extend the interface of the subtype.
abstract class $IS_EMPTY > $LIST, $SET is

5.4.1. Using supertyping

The main use of supertyping arises in defining appropriate type bounds for parametrized classes, and will be discussed in the next chapter (see unnamedlink).