Sat Apr 20 22:02:59 EDT 2019

Z-transform, summary

This will only work on linearized systems, so first create an autodiff
instance to be able to compute partial differentials.

So to implement autodiff, implement normal numbers.

EDIT: Almost done with that.  Rest is easy to fill in.

EDIT: I've re-invented rai/ai-freq.rkt

;; Basic strategy:
;; - normal eval: eval parent semantics lifted over small signals (linear approx)
;; - feedback: 
;;   - split variables in parameter / signal
;;   - compute output offsets
;;   - compute differential matrix of update function
;;   - return (memoized) z-dependent signals
;;     - compute z-dependent transfer function from z, matrix
;;     - apply transfer function

      ;; Obtain the frequency response of a linear system by first
      ;; probing the function for the linear system matrix, and then
      ;; compute the effect of feedback through matrix inversion
      ;; (solve linear system).