Specifies a constraint applied to a set of generalizations with the same
discriminator and the same parent, indicating that any instance of the
parent must be an instance of at least one child within the set of
generalizations. If a parent has a single discriminator, the set of its child
generalizations being complete implies that the parent is abstract. The
connotation of declaring a set of generalizations complete is that all of the
children with the given discriminator have been declared and that
additional ones are not expected (in other words, the set of generalizations
is closed), and designs may assume with some confidence that the set of
children is fixed. If a new child is nevertheless added in the future, existing
models may be adversely affected and may require modification.