& Example 2: fractal (binary) tree
& from https://en.wikipedia.org/wiki/L-system
box states = []

fn push! () -> {
  update! (states, append (_, turtle_state ()))
}

fn pop! () -> {
  let state = do states > unbox > last
  update! (states, butlast)
  penup! ()
  load_turtle_state! (state)
}

fn branch! () -> {
  pc! (colors :olive)
  pw! (4)
  fd! (20)
}

fn one! {
  (0) -> {
    branch! ()
  }
  (n) -> {
    one! (dec (n))
    one! (dec (n))
  }
}

fn zero! {
  (0) -> branch! ()
  (n) -> {
    one! (dec (n))
    push! ()
    lt! (inv (10))
    zero! (dec (n))
    pop! ()
    rt! (inv (10))
    zero! (dec (n))
  }
}

zero! (7)

hideturtle! ()