You may call v the density (time-dependent) of 1 in the flow (hence 1-v is the density of 0). Now you simply assume the flow is a differentiable function. As a

You may call v the density (time-dependent) of 1 in the flow (hence 1-v is the density of 0). Now you simply assume the flow is a differentiable function. As a matter of fact, it will ressemble reality much better than many economic models, for example (if not all): there is a HUGE amount of bits per second.