code: function PowerRadix (digits, sourceRadix) {
    sourceRadix = Array.isArray(sourceRadix) ? sourceRadix : B62.slice(0, sourceRadix);
    this._digits = Array.isArray(digits) ? digits : (digits+'').split('');
    this._sourceRadixLength = new BigInt(sourceRadix.length+'');
    this._sourceRadixMap = sourceRadix.reduce(function (map, char, i) {
      map[char+''] = new BigInt(i+'');
      return map;
    }, {});
  }
docs: Creates a new instance of PowerRadix
  @class
  @throws {InvalidArgumentException}
  @param {Array|String} digits
  @param {Array|Number} sourceRadix