code: public List<BType> getBuilderList() {
      if (externalBuilderList == null) {
        externalBuilderList = new BuilderExternalList<MType, BType, IType>(this);
      }
      return externalBuilderList;
    }
docs: Gets a view of the builder as a list of builders. This returned list is live and will reflect
  any changes to the underlying builder.
  
  @return the builders in the list