code: Stream<Casting> castingsInstance() {
          return vertex().getEdgesOfType(Direction.IN, Schema.EdgeLabel.ROLE_PLAYER).
                  map(edge -> Casting.withThing(edge, this));
      }
docs: Castings are retrieved from the perspective of the Thing which is a role player in a Relation
  
  @return All the Casting which this instance is cast into the role