Exercise 3.79.  Generalize the solve-2nd  procedure of exercise 3.78  so 
that it can be used to solve general second-order differential equations 
d2 y/dt2  = f(dy/dt, y).

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(define (solve-2nd f a b dt y0 dy0)
  (define y (integral (delay dy) y0 dt))
  (define dy (integral (delay ddy) dy0 dt))
  (define ddy (stream-map f dy y))
  y)