public void Append(System.UInt64 value, string format)
{
if (!value.TryFormat(buffer.AsSpan(index), out var written, format.AsSpan()))
{
Grow();
if (!value.TryFormat(buffer.AsSpan(index), out written, format.AsSpan()))
{
ThrowArgumentException(nameof(value));
}
}
index += written;
}