protected CmmnSentryPart createOnPart(
      CmmnSentryDeclaration sentryDeclaration, CmmnOnPartDeclaration onPartDeclaration) {
    CmmnSentryPart sentryPart = createSentryPart(sentryDeclaration, PLAN_ITEM_ON_PART);

    // set the standard event
    String standardEvent = onPartDeclaration.getStandardEvent();
    sentryPart.setStandardEvent(standardEvent);

    // set source case execution
    CmmnActivity source = onPartDeclaration.getSource();
    ensureNotNull(
        "The source of sentry '" + sentryDeclaration.getId() + "' is null.", "source", source);

    String sourceActivityId = source.getId();
    sentryPart.setSource(sourceActivityId);

    // TODO: handle also sentryRef!!! (currently not implemented on purpose)

    return sentryPart;
  }