old_contents: # test bytes + other
  
  print(b"123" + b"456")
  print(b"123" + bytearray(2))
  
new_contents: # test bytes + other
  
  print(b"123" + b"456")
  print(b"123" + bytearray(2))
  
  print(b"123" + b"") # RHS is empty, can be optimised
  print(b"" + b"123") # LHS is empty, can be optimised
  print(b"" + bytearray(1)) # LHS is empty but can't be optimised
  
subject: Add tests for optimised bytes addition.