design
  cons module Swap
    nil
    cons state a 1 false
    cons state b 1 true
    cons next a b
    cons next b a
    nil
    cons output first 1 a cons output second 1 b nil
  nil
  Swap
  cons test simultaneous Swap
    cons tick nil cons false cons true nil
    cons tick nil cons true cons false nil
    cons tick nil cons false cons true nil
    nil
  nil