/**
 * 字符串递增
 * @param  {string} prefix         字符串前缀，若无前缀则传''
 * @param  {number} digit          位数，从1开始
 * @example
 * 	// todo: need unit tests
 * 	feidao.str_increasing('S01', 'S', 2);//S02
 * 	feidao.str_increasing('01', '', 3);//002
 */
export default function increasing(str: string, prefix: string, digit: number): string;