KERML — Features successors and predecessors of Occurrence should be disjoint

Features successors and predecessors of Occurrence should be disjoint. Otherwise, the library would allow circular HappensBefore graphs. If HappensBefore is to be interpreted as a strict temporal ordering constraint (which is suggested by the current documentation in the library) or is supposed to capture causality relationships, such cycles could/should not be collapsed into time-coincident occurrences, always leading to unsatisfiable models.