We may represent these constraints by saying that these nodes are periodic events to be scheduled and the arcs are constraints for particular pairs of events. A possible way to solve it is by first finding a spanning tree for these arcs and then determining integral variables for the chords of the spanning tree. The enumeration tree has the purpose of determining the integer values and the nice thing is that in each node of the enumeration tree what we have to find out is a shortest path. Otherwise we may resort to the previous formulation which was after all linear programming and we may choose either model to resolve the problem.