docs: function(category, title, ts, opt_args, opt_tts,
                           opt_argsStripped, opt_colorId) {
        if (this.openPartialSlices_.length) {
          var prevSlice = this.openPartialSlices_[
              this.openPartialSlices_.length - 1];
          if (ts < prevSlice.start)
            throw new Error('Slices must be added in increasing timestamp order');
        }
  
        var colorId = opt_colorId ||
            ColorScheme.getColorIdForGeneralPurposeString(title);
        var slice = new this.sliceConstructor(category, title, colorId, ts,
                                              opt_args ? opt_args : {}, null,
                                              opt_tts, undefined,
                                              opt_argsStripped);
        this.openPartialSlices_.push(slice);
        slice.didNotFinish = true;
        this.pushSlice(slice);
  
        return slice;
      }