cfiFromLocation(loc) {
    var cfi = -1; // check that pg is an int

    if (typeof loc != "number") {
      loc = parseInt(loc);
    }

    if (loc >= 0 && loc < this._locations.length) {
      cfi = this._locations[loc];
    }

    return cfi;
  }
Get an EpubCFI from location index
@param {number} loc
@return {EpubCFI} cfi