Map different relationship types to one collection

Question

I am trying to map ontology terms to a Neo4j data-structure.
To be precise I have a Term class and 2 types of relationships (is_a and part_of). I have following classes:

Term class:

@NodeEntity
public class Term {
    @GraphId
    private Long nodeId;
    @Indexed(unique=true)
    private String id;
    private String name;
    private String definition;

    @RelatedToVia(type="is_a",direction=Direction.INCOMING)
    private Set<Term2Term> is_a_children;

    @RelatedToVia(type="is_a",direction=Direction.OUTGOING)
    Set<Term2Term> is_a_parents;

    @RelatedToVia(type="part_of",direction = Direction.INCOMING)
    Set<Term2Term> part_of_children;

    @RelatedToVia(type="part_of",direction = Direction.OUTGOING)
    Set<Term2Term> part_of_parents;
}

Term2Term class:

@RelationshipEntity
public class Term2Term {

    @GraphId
    Long id;
    @StartNode private Term child;
    @EndNode private Term parent;
}

The mapping works fine and accessing the specific collections (i.e. is_a_children) works fine. However I have a use case where I have to populate a NavigationTree with the data. It should look something like this:

-Term 1 
  - Term 1.1 (is_a)
     -Term 1.1.1 (part_of)
  - Term 1.2 (part_of)
  - Term 1.3 (is_a)
     - Term 1.3.1 (part_of)
     - Term 1.3.2. (is_a)
....

The NavigationalTree has no notion of the different relationship types.

I need to combine the different relationship collections (i.e. is_a_children and part_of_children) to one collection and still keep the information about the type of relationship (in order to display it)

Of course I can create a getter which does a Sets.union (guava) of the two sets, however this way I lose the information about the relationship type. The only non-hackish way would be to use one generic relationship type instead of the two relationship types (is_a,part_of) in the graph database and add the type as a property to the relationship.

Is there any other way?

Update 1:

It seems that I have to use inheritence to solve it Spring Data Neo4j - @RelationshipType issues
http://forum.springsource.org/showthread.php?124110-neo4j-Inheritance-with-RelationshipEntities&highlight=neo4j+inheritance+with+relationshipentities