private async Task <int> ReadIntoSecondaryAfterTeeWriteIsFinished(byte[] buf)
{
await TeeWrite; // we can't reuse the buffer as long as it isn't fully written
int read = await Underlying.ReadAsync(buf, 0, buf.Length); // alright, now let's read in some data
TeeWrite = Additional.WriteAsync(buf, 0, read);
return(read);
}