/**
 * Computes the substring of a given string, conditionally indexing from the end of the string.
 * When the string is long enough to fully include the substring, return the substring.
 * Otherwise, return None. The start index is inclusive and the stop index is exclusive.
 * The length of the returned string will always be stop-start.
 */
export declare const substring: (input: string, start: number, stop: number, reverse: boolean) => string | null;