(VAR y x f g)
(RULES
  app(app(plus, 0), y) -> y
  app(app(plus, app(s, x)), y) -> app(s, app(app(plus, x), y))
  app(app(times, 0), y) -> 0
  app(app(times, app(s, x)), y) -> app(app(plus, app(app(times, x), y)), y)
  app(app(app(comp, f), g), x) -> app(f, app(g, x))
  app(twice, f) -> app(app(comp, f), f)
)