(* SIGNATURES *)
module type VERTEX =
sig
  type t
  type label

  val equal : t -> t -> bool
  val create : label -> t
  val label : t -> label

end

module type EDGE =
sig
  type t
  type v
  type label

  val equal : t -> t -> bool
  val create : v -> v -> label -> t
  val label : t -> label
  val in_v : t -> v
  val out_v : t -> v
end

module type GRAPH =
sig
  type t
  type v
  type e

  val init : unit -> t
  val insert_v : t -> v -> t
  val insert_e : t -> e -> t
  val fold_v : (v -> 'a -> 'a) -> t -> 'a -> 'a
end

(* STRUCTS *)
module Vertex : VERTEX with type label = int =
struct
  type t = int
  type label = int

  let equal v1 v2 = (v1 = v2)
  let create l = l
  let label l = l
end


module AbstractEdge(Vertex : VERTEX) : (EDGE with type v = Vertex.t and type label = string) =
struct
  type v = Vertex.t
  type label = string
  type t = v * v * label

  let fst (x, _, _) = x
  let snd (_, x, _) = x
  let trd (_, _, x) = x

  let equal e1 e2 = ((fst e1) = (fst e2)) && ((snd e1) = (snd e2)) && ((trd e1) = (trd e2))
  let create v1 v2 l = v1, v2, l
  let label e = trd e
  let in_v e = fst e
  let out_v e = snd e

end

module Edge = AbstractEdge(Vertex)