/**
   * @param e the event to check for violation metrics.
   * @param filterUnits
   * @return the particular monitored metric violation or <code>null</code>, or <code>null</code>.
   */
  Violation isViolated(final MonitoringEvent e, final List<String> filterUnits) {
    final double observedValue = (Double) e.get(metric);
    final boolean res = pred.perform(observedValue, thresholdValue);

    if (!res) return null;

    final ArrayList<String> applicable = new ArrayList<String>(filterUnits.size());

    for (final String filterUnit : filterUnits)
      if (ThresholdRulesTraits.join(e).startsWith(filterUnit)) applicable.add(filterUnit);

    return new Violation(metric, thresholdValue, observedValue, applicable);
  }
  /**
   * @param events the list of events to check for violation metrics.
   * @param filterUnits
   * @return the particular monitored metric violation or <code>null</code>, or <code>null</code>.
   */
  Violation isViolated(final List<MonitoringEvent> events, final List<String> filterUnits) {
    // FIXME: should check specific filterUnit
    final double observedValue = performFold(events);
    final boolean res = pred.perform(observedValue, thresholdValue);

    if (!res) return null;

    final ArrayList<String> applicable = new ArrayList<String>(filterUnits.size());

    for (final String filterUnit : filterUnits)
      for (final MonitoringEvent e : events)
        if (ThresholdRulesTraits.join(e).startsWith(filterUnit)) applicable.add(filterUnit);

    return new Violation(metric, thresholdValue, observedValue, null);
  }