Graphical Modeling Framework Tooling - Eclipse Modeling Project: A Domain-Specific Language Toolkit - page 521

11.3.2 Top Node Reference

A Top Node Reference represents elements that are created on the diagram
surface. This element contains a Child reference to a single Node Mapping .
Elements must be contained somewhere in the corresponding domain model
when they are instantiated as diagram elements. The Containment Feature
property specifies where to add these new objects and, by default, where to
retrieve them.
A common pattern in modeling involves having a single containment refer-
ence for a generic type that is used to store model objects, while a derived attrib-
ute of a specific type is used to retrieve objects. As an example, consider the
mindmap domain model used in this topic, shown again in Figure 11-8 for
convenience.

Figure 11-8 Mindmap model

Here, an elements containment reference is used to hold all MapElement
objects in the Map . To obtain a list of root Topic objects, a derived rootTopics
reference is provided and implemented using OCL (refer back to Section 3.3.5,
“Adding OCL”).
In some models, you might find it necessary to specify both the
Containment Feature and a Children Feature for a Top Node