> `-Parallel(tf1, tf2)` to return `Series(TransferFunction(-1, 1, s), Parallel(tf1, tf2))`    Yes, I was thinking of the same approach. Also, `tf1*tf2 - tf3*tf4` would return `Parallel(Series(tf1, tf2), -Series(tf3, tf4))`. Since we don't want anything like `Series(tf, Series(...))`, `-Series(tf3, tf4)` would be left as it is.