Exercise 3.73: We can model electrical circuits
using streams to represent the values of currents or voltages at a sequence of
times.  For instance, suppose we have an 
RC circuit consisting of a
resistor of resistance 
  R
 and a capacitor of capacitance 
  C
 in series.
The voltage response 
  v
 of the circuit to an injected current 
  i
 is
determined by the formula in Figure 3.33, whose structure is shown by the
accompanying signal-flow diagram.



SVG


Figure 3.33: An RC circuit and the associated signal-flow diagram.



Write a procedure RC that models this circuit.  RC should take as
inputs the values of 
  R
, 
  C
, and 
  
    d
    t
  
 and should return a procedure
that takes as inputs a stream representing the current 
  i
 and an initial
value for the capacitor voltage 
  
    v
    0
  
 and produces as output the stream of
voltages 
  v
.  For example, you should be able to use RC to model an
RC circuit with 
  R
 = 5 ohms, 
  C
 = 1 farad, and a 0.5-second time step by
evaluating (define RC1 (RC 5 1 0.5)).  This defines RC1 as a
procedure that takes a stream representing the time sequence of currents and an
initial capacitor voltage and produces the output stream of voltages.