code: function (subSection) {
                  this.$assert(141, !subSection.parent);
                  this.$assert(142, subSection._type == TYPE_SECTION);
                  this.$assert(143, subSection.tplCtxt == this.tplCtxt);
                  subSection.parent = this;
                  subSection.__updateIdMap(this.idMap);
                  this._content.push(subSection);
              }
docs: It is more efficient to have an empty or small sub-section. The size of the current section or its
  ancestors has no impact. addSubSection add elements from the subsection to its parent map (used for
  retrieving elements) and replace map of subsection
  @param {aria.templates.Section} subSection