type var = int

type t =
  | Var of var
  | Const of string
  | Free of string
  | Function of string * t list

type match_result = All of t | Some of t | None | MatchErr

val merge_result : match_result -> match_result -> match_result
val match_term : t -> t -> t -> match_result
val inc_var : t -> t
val occurs : t -> t -> bool
val to_string : string list -> t -> string