code: public int evaluate(Model mo, Class<? extends Action> a, Element e) throws SchedulerException {
          ActionDurationEvaluator<Element> ev = durations.get(a);
          if (ev == null) {
              throw new SchedulerModelingException(null, "Unable to estimate the duration of action '" + a.getSimpleName() + "' related to '" + e + "'");
          }
          int d = ev.evaluate(mo, e);
          if (d <= 0) {
              throw new SchedulerModelingException(null, "The duration for action " + a.getSimpleName() + " over '" + e + "' has been evaluated to a negative value (" + d + "). Unsupported");
          }
          return d;
      }
docs: Evaluate the duration of given action on a given element.
  
  @param mo the model to consider
  @param a  the action' class
  @param e  the element identifier
  @return a positive number if the evaluation succeeded. A negative number otherwise