code: function insertPart(summary, part, index) {
      part = SummaryPart(part);
  
      var parts = summary.getParts().insert(index, part);
      return indexLevels(summary.set('parts', parts));
  }
docs: Returns a new Summary with a part inserted at given index
  
  @param {Summary} summary
  @param {Part} part
  @param {Number} index
  @return {Summary}