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

    if (typeof pg != "number") {
      pg = parseInt(pg);
    } // check if the cfi is in the page list
    // Pages could be unsorted.


    var index = this.pages.indexOf(pg);

    if (index != -1) {
      cfi = this.locations[index];
    } // TODO: handle pages not in the list


    return cfi;
  }
Get an EpubCFI from a Page List Item
@param  {string | number} pg
@return {string} cfi