## 7.3 HierarchicalReferences ReferenceType

The HierarchicalReferences ReferenceType is an abstract ReferenceType; only subtypes of it can be used and they shall be a non-symmetric Reference.

The semantic of HierarchicalReferences is to denote that References of HierarchicalReferences span a hierarchy. It means that it may be useful to present Nodes related with References of this type in a hierarchical-like way. HierarchicalReferences does not forbid loops. For example, starting from Node “A” and following HierarchicalReferences it may be possible to browse to Node “A”, again.

It is not permitted to have a Property as SourceNode of a Reference of any subtype of this abstract ReferenceType.

It is not allowed that the SourceNode and the TargetNode of a Reference of the ReferenceType HierarchicalReferences are the same, that is, it is not allowed to have self-references using HierarchicalReferences.