code: public List<Relationship> getPrerequisiteRelationships() {
          final List<Relationship> prerequisiteRelationships = new LinkedList<Relationship>();
          for (final Relationship r : relationships) {
              if (r.getType() == RelationshipType.PREREQUISITE) {
                  prerequisiteRelationships.add(r);
              }
          }
          return prerequisiteRelationships;
      }
docs: Gets a list of prerequisite relationships for the topic.