function _bi(value, base) {
        if (base !== undefined) {
            if (base === 256) {
                return _bi(utils.string2bytes(value));
            }
            return new BigInteger(value, base);
        } else if (typeof value === 'string') {
            return new BigInteger(value, 10);
        } else if ((value instanceof Array) || (value instanceof Uint8Array)
          || Buffer.isBuffer(value)) {
            return new BigInteger(value);
        } else if (typeof value === 'number') {
            return new BigInteger(value.toString(), 10);
        } else {
            throw "Can't convert " + value + " to BigInteger";
        }
    }
Factory function to create a suitable BigInteger.

@param value
    The value for the big integer.
@param base {integer}
    Base of the conversion of elements in ``value``.
@returns
    A BigInteger object.