From Undecidability Require Import Switch.

Section Composition.

  Variable n : nat.
  Variable sig : finType.


  Variable F1 : finType.
  Variable pM1 : pTM sig F1 n.

  Variable F2 : finType.
  Variable pM2 : pTM sig F2 n.

  Definition Seq := Switch pM1 (fun _ => pM2).

  Lemma Seq_Realise R1 R2 :
    pM1 ⊨ R1 ->
    pM2 ⊨ R2 ->
    Seq ⊨ ⋃_y ((R1 |_ y) ∘ R2).
  Proof.
    intros.
    eapply Realise_monotone.
    eapply (Switch_Realise (R1 := R1) (R2 := (fun _ => R2))); eauto.
    firstorder.
  Qed.

  Lemma Seq_TerminatesIn R1 T1 T2 :
    pM1 ⊨ R1 ->
    projT1 pM1 ↓ T1 ->
    projT1 pM2 ↓ T2 ->
    projT1 Seq ↓
           (fun tin i => exists i1 i2, T1 tin i1 /\ 1 + i1 + i2 <= i /\
                               forall tout yout, R1 tin (yout, tout) -> T2 tout i2).
  Proof.
    intros HRealise HTerm1 HTerm2.
    eapply TerminatesIn_monotone.
    - eapply Switch_TerminatesIn; eauto. instantiate (1 := fun _ => T2). auto.
    - intros tin i (i1&i2&Hi&HT1&HT2). exists i1, i2; repeat split; auto.
  Qed.

  Lemma Seq_RealiseIn (R1 : Rel _ _) (R2 : Rel _ (F2 * _)) k1 k2:
    pM1 ⊨c(k1) R1 ->
    pM2 ⊨c(k2) R2 ->
    Seq ⊨c(1 + k1 + k2) ⋃_y ((R1 |_y) ∘ R2).
  Proof.
    intros H1 H2.
    eapply RealiseIn_monotone.
    eapply (Switch_RealiseIn).
    - eapply H1.
    - intros f. eapply H2.
    - omega.
    - firstorder.
  Qed.

End Composition.

Notation "A ;; B" := (Seq A B) (right associativity, at level 65).

Arguments Seq : simpl never.