type t =
  | Var of int
  | Free of string
  | Lam of t
  | App of t * t

module SSet : Set.S with type elt = string

exception Unbound_index of int

val free_variables : t -> SSet.t
val free_variable_list : t -> string list
val pretty : t -> string
val shift : int -> int -> t -> t
val subst : int -> t -> t -> t
val substitute : int -> t -> t -> t
val convert_from_named : Named.t -> t
val convert_to_named : t -> Named.t
val alpha_equivalent : t -> t -> bool
val alpha_equivalent_counted : t -> t -> bool * int * int
val normalise : t -> t
val normalise_counted : t -> t * int * int
val well_scoped : int -> t -> bool