type t =
  | BVar of int
  | FVar of string
  | Lam of t
  | App of t * t

module SSet : Set.S with type elt = string

exception Not_locally_closed

val free_variables : t -> SSet.t
val free_variable_list : t -> string list
val pretty : t -> string
val open_at : int -> string -> t -> t
val open_top : t -> string -> t
val close_at : int -> string -> t -> t
val close_top : t -> string -> t
val lc : t -> bool
val require_lc : t -> unit
val subst_bvar : int -> t -> t -> t
val subst_fvar : string -> t -> t -> t
val substitute : string -> 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