(VAR x y p f)
(RULES
  app(app(app(if, true), x), y) -> x
  app(app(app(if, false), x), y) -> y
  app(app(app(until, p), f), x) -> app(app(app(if, app(p, x)), x), app(app(app(until, p), f), app(f, x)))
)