code: private IntVar makeDuration() throws SchedulerException {
          if (start.isInstantiated() && end.isInstantiated()) {
              int d = end.getValue() - start.getValue();
              return rp.makeDuration(d, d, lblPrefix, "_duration");
          } else if (start.isInstantiated()) {
              if (start.isInstantiatedTo(0)) {
                  return end;
              }
              return rp.getModel().intOffsetView(end, -start.getValue());
          }
          int inf = end.getLB() - start.getUB();
          if (inf < 0) {
              inf = 0;
          }
          int sup = end.getUB() - start.getLB();
          return rp.makeDuration(sup, inf, lblPrefix, "_duration");
      }
docs: Make the duration variable depending on the others.