transformBreak(labelSet, breakState = undefined) {
    if (this.label === null)
      return new FallThroughState(this.id, breakState, []);

    if (labelSet.has(this.label)) {
      return new FallThroughState(this.id,
          labelSet.get(this.label).fallThroughState, []);
    }

    return this;
  }
@param {StringMap} labelSet
@param {number=} breakState
@return {State}