code: function multiply(str, multiplier, scale) {
      var res = [];
      var carry = 0;
      var i = str.length;
      var n;
      while(--i >= 0) {
        n = hexNumber(str[i]) * multiplier + carry;
        carry = Math.floor(n / scale);
        res.push(n % scale);
      }
      while(carry > 0) {
        res.push(carry % scale);
        carry = Math.floor(carry / scale);
      }
      return res.reverse().join('');
    }
docs: /*
  @desc multiply hex string
  @params {String} hex string multiplicand
  @params {Number} multiplier
  @params {Number} scale
  @return {String} multiplied hex string