docs: function fromAlphabet(str, alphabet) {
  				var base = alphabet.length;
  				var pos = 0;
  				var num = 0;
  
  				for (let chr of str) {
  					let at = alphabet.indexOf(chr);
  					if (at === -1) return NaN; // 因为返回的类型都是数字，所以这里直接返回 NaN
  					num += Math.pow(base, pos) * at;
  					pos++;
  				}
  
  				return num;
  			}