namespace System.Numerics
{
    public struct BigInteger
    {
        public BigInteger(ReadOnlySpan<byte> value);

        public int GetByteCount();
        public bool TryWriteBytes(Span<byte> destination, out int bytesWritten);

        public static BigInteger Parse(ReadOnlySpan<char> value, NumberStyles style = NumberStyles.Integer, IFormatProvider provider = null);
        public static bool TryParse(ReadOnlySpan<char> value, out BigInteger result, NumberStyles style = NumberStyles.Integer, IFormatProvider provider = null);

        public static bool TryFormat(Span<char> destination, out int charsWritten, string format = null, IFormatProvider provider = null);
        …
    }
}