The Drupal module LinearCase allows ClosedQuestions and other documents to be ordered in a "case study". The user is not allowed to navigate further than the last question he has not yet answered correctly, but is allowed to navigate backwards freely.
Relation is an API module and storage model for both simple and the most complex relations between entities. The module can handle both directional and symmetrical relations very well.
Relations are entities and can therefore have fields. This makes Relation the most flexible and powerful relation model out there. (Still, using Relation with anything but the default SQL storage will not work in most cases.)