design
  cons module Register
    cons input d 1 nil
    cons state q 1 false
    cons next q d
    nil
    cons output value 1 q nil
  cons module Feedback
    nil
    cons inst reg Register cons not at reg value nil
    nil
    cons output value 1 at reg value nil
  nil
  Feedback
  cons test feedback Feedback
    cons tick nil cons false nil
    cons tick nil cons true nil
    cons tick nil cons false nil
    nil
  nil