From Undecidability Require Import FOL.Util.Tarski FOL.Util.Syntax.
Import FragmentSyntax.
From Undecidability Require Import Shared.ListAutomation.
Import ListAutomationNotations.
Local Set Implicit Arguments.
Ltac comp := repeat (progress (cbn in *; autounfold in *)).
Inductive peirce := class | intu.
Existing Class peirce.
Section ND_def.
Context {Σ_funcs : funcs_signature}.
Context {Σ_preds : preds_signature}.
Reserved Notation "A ⊢ phi" (at level 61).
Implicit Type p : peirce.
Implicit Type ff : falsity_flag.
Inductive prv : forall (ff : falsity_flag) (p : peirce), list form -> form -> Prop :=
| II {ff} {p} A phi psi : phi::A ⊢ psi -> A ⊢ phi --> psi
| IE {ff} {p} A phi psi : A ⊢ phi --> psi -> A ⊢ phi -> A ⊢ psi
| AllI {ff} {p} A phi : map (subst_form ↑) A ⊢ phi -> A ⊢ ∀ phi
| AllE {ff} {p} A t phi : A ⊢ ∀ phi -> A ⊢ phi[t..]
| Exp {p} A phi : prv p A falsity -> prv p A phi
| Ctx {ff} {p} A phi : phi el A -> A ⊢ phi
| Pc {ff} A phi psi : prv class A (((phi --> psi) --> phi) --> phi)
where "A ⊢ phi" := (prv _ A phi).
Arguments prv {_} _ _.
Context {ff : falsity_flag}.
Context {p : peirce}.
Lemma impl_prv A B phi :
(rev B ++ A) ⊢ phi -> A ⊢ (B ==> phi).
Proof.
revert A; induction B; intros A; cbn; simpl_list; intros.
- firstorder.
- eapply II. now eapply IHB.
Qed.
Theorem Weak A B phi :
A ⊢ phi -> A <<= B -> B ⊢ phi.
Proof.
intros H. revert B.
induction H; intros B HB; try unshelve (solve [econstructor; intuition]); try now econstructor.
Qed.
End ND_def.
Hint Constructors prv : core.
Arguments prv {_ _ _ _} _ _.
Notation "A ⊢ phi" := (prv A phi) (at level 30).
Notation "A ⊢C phi" := (@prv _ _ _ class A phi) (at level 30).
Notation "A ⊢I phi" := (@prv _ _ _ intu A phi) (at level 30).
Notation "A ⊢M phi" := (@prv _ _ falsity_off intu A phi) (at level 30).
Section Soundness.
Context {Σ_funcs : funcs_signature}.
Context {Σ_preds : preds_signature}.
Lemma soundness {ff : falsity_flag} A phi :
A ⊢I phi -> valid_ctx A phi.
Proof.
remember intu as p.
induction 1; intros D I rho HA; comp.
- intros Hphi. apply IHprv; trivial. intros ? []; subst. assumption. now apply HA.
- now apply IHprv1, IHprv2.
- intros d. apply IHprv; trivial. intros psi [psi'[<- H' % HA]] % in_map_iff.
eapply sat_comp. now comp.
- eapply sat_comp, sat_ext. 2: apply (IHprv Heqp D I rho HA (eval rho t)). now intros [].
- apply (IHprv Heqp) in HA. firstorder.
- firstorder.
- discriminate.
Qed.
Lemma soundness' {ff : falsity_flag} phi :
[] ⊢I phi -> valid phi.
Proof.
intros H % soundness. firstorder.
Qed.
End Soundness.