(** Some fresh variables *)
Notation X := (fresh nil).Notation Y := (fresh (X :: nil)).Notation Z := (fresh (X :: Y :: nil)).(*************************************************************)
(** * A nominal representation of terms                      *)
(*************************************************************)

Inductive n_exp : Set :=
 | n_var (x:atom)
 | n_abs (x:atom) (t:n_exp)
 | n_app (t1:n_exp) (t2:n_exp).(*************************************************************)
(** * A nominal representation of terms                      *)
(*************************************************************)

Inductive n_exp : Set :=
 | n_var (x:atom)
 | n_abs (x:atom) (t:n_exp)
 | n_app (t1:n_exp) (t2:n_exp).(*************************************************************)
(** * A nominal representation of terms                      *)
(*************************************************************)

Inductive n_exp : Set :=
 | n_var (x:atom)
 | n_abs (x:atom) (t:n_exp)
 | n_app (t1:n_exp) (t2:n_exp).(*************************************************************)
(** * A nominal representation of terms                      *)
(*************************************************************)

Inductive n_exp : Set :=
 | n_var (x:atom)
 | n_abs (x:atom) (t:n_exp)
 | n_app (t1:n_exp) (t2:n_exp).(** For example, we can encode the expression [(\X.Y X)] as below.  *)

Definition demo_rep1 := n_abs X (n_app (n_var Y) (n_var X)).(** For example, we can encode the expression [(\Z.Y Z)] as below.  *)

Definition demo_rep2 := n_abs Z (n_app (n_var Y) (n_var Z)).(** The main insight of nominal representations is that we can
    rename variables, without capture, using a simple
    structural induction. Note how in the [n_abs] case we swap
    all variables, both bound and free.

    For example:

      (swap x y) (\z. (x y)) = \z. (y x))

      (swap x y) (\x. x) = \y.y

      (swap x y) (\y. x) = \x.y

*)
Fixpoint swap (x:atom) (y:atom) (t:n_exp) : n_exp :=
  match t with
  | n_var z     => n_var (swap_var x y z)
  | n_abs z t1  => n_abs (swap_var x y z) (swap x y t1)
  | n_app t1 t2 => n_app (swap x y t1) (swap x y t2)
  end.(** Because swapping is a simple, structurally recursive
    function, it is highly automatable using the [default_simp]
    tactic from LNgen library.

    This tactic "simplifies" goals using a combination of
    common proof steps, including case analysis of all disjoint
    sums in the goal. Because atom equality returns a sumbool,
    this makes this tactic useful for reasoning by cases about
    atoms.

    For more information about the [default_simp] tactic, see
    [metalib/LibDefaultSimp.v].

    WARNING: this tactic is not always safe. It's a power tool
    and can put your proof in an irrecoverable state. *)

Example swap1 : forall x y z, x <> z -> y <> z ->
    swap x y (n_abs z (n_app (n_var x)(n_var y))) = n_abs z (n_app (n_var y) (n_var x)).Example swap2 : forall x y, x <> y ->
    swap x y (n_abs x (n_var x)) = n_abs y (n_var y).Example swap3 : forall x y, x <> y ->
     swap x y (n_abs y (n_var x)) = n_abs x (n_var y).Example aeq1 : forall x y, x <> y -> aeq (n_abs x (n_var x)) (n_abs y (n_var y)).

Lemma aeq_equivariance : forall x y t1 t2, aeq t1 t2 -> aeq (swap x y t1) (swap x y t2).