I think the source of confusion was this. Daniel's point was that the initial representation of each hierarchy could be a DAG, since it is a generalization of a tree. Rich's point was that if you start out with all the hierarchies in whatever form, and you include defined hierarchies which do not include the Resource in question as a member, even though ancestors of the Resource are members of the hierarchy, if you combine all the hierarchies you lose the information about the original hierarchies necessary to be able to distinguish whether the nodes above the Resource are true ancestors or not.
