navItemFromCfi(cfi) {

		// This feature was added to solve the problem of duplicate titles in 
		// bookmarks. But this still has no solution because when reloading the 
		// reader, rendition cannot get the range from the previously saved CFI.
		const range = this.rendition.getRange(cfi);
		const idref = range ? range.startContainer.parentNode.id : undefined;
		const location = this.rendition.currentLocation();
		const href = location.start.href;
		return this.navItems[href + "#" + idref] || this.navItems[href];
	}
Verifying the current page in bookmarks.
@param {*} cfi
@returns The index of the bookmark if it exists, or -1 otherwise.