Exercise 3.78: Consider the problem of designing
a signal-processing system to study the homogeneous second-order linear
differential equation

  
    
      
        d
        2
      
      y
    
    
      d
      
        t
        2
      
    
  
  −
  
    a
    
      
        d
        y
      
      
        d
        t
      
    
  
  −
  
    b
    y
  
  
  =
  
  
    0.
  

The output stream, modeling 
  y
, is generated by a network that contains a
loop. This is because the value of 
  
    
      d
      2
    
    y
    
      /
    
    d
    
      t
      2
    
  
 depends upon the
values of 
  y
 and 
  
    d
    y
    
      /
    
    d
    t
  
 and both of these are determined by
integrating 
  
    
      d
      2
    
    y
    
      /
    
    d
    
      t
      2
    
  
.  The diagram we would like to encode is
shown in Figure 3.35.  Write a procedure solve-2nd that takes as
arguments the constants 
  a
, 
  b
, and 
  
    d
    t
  
 and the initial values

  
    y
    0
  
 and 
  
    d
    
      y
      0
    
  
 for 
  y
 and 
  
    d
    y
    
      /
    
    d
    t
  
 and generates the
stream of successive values of 
  y
.