design
  cons module HalfAdder
    cons input a 1 cons input b 1 nil
    nil
    cons output sum 1 xor a b
    cons output carry 1 and a b
    nil
  nil
  HalfAdder
  cons test all_inputs HalfAdder
    cons tick cons false cons false nil cons false cons false nil
    cons tick cons false cons true nil cons true cons false nil
    cons tick cons true cons false nil cons true cons false nil
    cons tick cons true cons true nil cons false cons true nil
    nil
  nil