In the context of logic, the usual way to capture these sorts of things is to introduce some notion of an event/eventuality which you can use to index everything by. Then you can say that there exists some index (e.g., hotel reservation) with such and such properties (e.g., consumes a free room, provides the same room to the reservation holder, etc). The connection between this semantic model and how you write your program is, of course, up to you. Both the pointer-indirection approach and the unification-variable approach can be seen as ways of implementing this semantics. As for a general concept, you may want to look into "constraint satisfaction"/"constraint programming" (e.g., ECLPS Prolog http://eclipseclp.org). The big idea here is that you can give each variable a domain of possible values (which need not be the same for all variables), and then you introduce a set of constraints on what those values can be. This allows for capturing things like "I have three reservations X, Y, and Z for three rooms A, B, and C such that no room is booked twice and no reservation takes more than one room". -- Live well, ~wren