code: protected String addPostRunDependent(TaskGroup.HasTaskGroup dependent) {
          Objects.requireNonNull(dependent);
          this.taskGroup.addPostRunDependentTaskGroup(dependent.taskGroup());
          return dependent.taskGroup().key();
      }
docs: Add a "post-run" dependent for this model.
  
  @param dependent the "post-run" dependent.
  @return key to be used as parameter to taskResult(string) method to retrieve result of root
  task in the given dependent task group