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;
 }