code: public static TriggerContext on(Trigger trigger) {
          List<Trigger> addedTriggers = new ArrayList<Trigger>();
          if (trigger != null) {
              addedTriggers.add(trigger);
          }
          return new TriggerContext(addedTriggers);
      }
docs: Adds the specified trigger to the validator under construction.
  
  @param trigger Trigger to be added.
  
  @return Context allowing further construction of the validator using the DSL.