new_contents: # Extended stream operations on io.BytesIO
  try:
      import uio as io
  except ImportError:
      import io
  
  a = io.BytesIO()
  print(a.seek(8))
  a.write(b"123")
  print(a.getvalue())
  
  print(a.seek(0, 1))
  
  print(a.seek(-1, 2))
  a.write(b"0")
  print(a.getvalue())
  
  a.flush()
  print(a.getvalue())
  
subject: Test for .seek()/.flush() on BytesIO.