code: public boolean register(Class<? extends Action> a, ActionDurationEvaluator e) {
          return durations.put(a, e) == null;
      }
docs: Register a new {@link ActionDurationEvaluator}.
  
  @param a the action class
  @param e the evaluator to register for the given action
  @return {@code false} if this action delete a previous evaluator for that action