exp() is supported for negative constant time s -- exp(-Bs) would be valid for B being a positive constant. It translates into a delay.
s = tf('s');
f = (s^2 - 1)/(s^3 + s - 1)
g = (s^2 + 1)/(s^3 + s + 1)
step(f)
step(g)
h = f * exp(-5*s) + g * exp(-10*s)
step(h)
