Before revision:  Often there is a real-world cost for each additional hidden state, and for each additional hidden timestep. We derive the associated " space/time " tradeoff between the numbers of hidden states and  of hidden timesteps needed to implement any given f(x) .
After revision:  Often there is a cost both for more hidden states and for more hidden timesteps. Accordingly, we derive a " space/time " tradeoff between the numbers of hidden states and of hidden timesteps needed to implement any given f(x) .