code: function SliceGroup(parentContainer, opt_sliceConstructor, opt_name) {
      tr.model.EventContainer.call(this);
  
      this.parentContainer_ = parentContainer;
  
      var sliceConstructor = opt_sliceConstructor || Slice;
      this.sliceConstructor = sliceConstructor;
  
      this.openPartialSlices_ = [];
  
      this.slices = [];
      this.topLevelSlices = [];
      this.haveTopLevelSlicesBeenBuilt = false;
      this.name_ = opt_name;
  
      if (this.model === undefined)
        throw new Error('SliceGroup must have model defined.');
    }
docs: A group of Slices, plus code to create them from B/E events, as
  well as arrange them into subRows.
  
  Do not mutate the slices array directly. Modify it only by
  SliceGroup mutation methods.
  
  @constructor
  @param {function(new:Slice, category, title, colorId, start, args)=}
  opt_sliceConstructor The constructor to use when creating slices.
  @extends {tr.model.EventContainer}