datatype list(a: t@ype+) =
  | cons of (a, list(a))
  | nil

fun concat {a:t@ype+} (list(a), list(a)) : list(a)

typedef digit(a: t@ype+) = list(a)

datatype node(a: t@ype) =
  | node2 of (a, a)
  | node3 of (a, a, a)

datatype finger_tree(a: t@ype+) =
  | empty
  | single of (a)
  | deep of (digit(a), finger_tree(node(a)), digit(a))

symintr <| |>

infixr 5 <|
infixl 5 |>

fun lcons {a:t@ype+} (a, finger_tree(a)) : finger_tree(a)

fun rcons {a:t@ype+} (finger_tree(a), a) : finger_tree(a)

overload <| with lcons
overload |> with rcons