Theory PropLog (Isabelle2005: October 2005)

# Theory PropLog

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theory PropLog
imports Main
begin

    (*  Title:      ZF/Induct/PropLog.thy
        ID:         $Id: PropLog.thy,v 1.4 2005/06/17 14:15:11 haftmann Exp $
        Author:     Tobias Nipkow & Lawrence C Paulson
        Copyright   1993  University of Cambridge
    *)

    header {* Meta-theory of propositional logic *}

    theory PropLog imports Main begin

    text {*
      Datatype definition of propositional logic formulae and inductive
      definition of the propositional tautologies.

      Inductive definition of propositional logic.  Soundness and
      completeness w.r.t.\ truth-tables.

      Prove: If @{text "H |= p"} then @{text "G |= p"} where @{text "G ∈
      Fin(H)"}
    *}

    subsection {* The datatype of propositions *}

    consts
      propn :: i

    datatype propn =
        Fls
      | Var ("n ∈ nat")    ("#_" [100] 100)
      | Imp ("p ∈ propn", "q ∈ propn")    (infixr "=>" 90)

    subsection {* The proof system *}

    consts thms     :: "i => i"
    syntax "_thms"  :: "[i,i] => o"    (infixl "|-" 50)
    translations "H |- p" == "p ∈ thms(H)"

    inductive
      domains "thms(H)" ⊆ "propn"
      intros
        H:  "[| p ∈ H;  p ∈ propn |] ==> H |- p"
        K:  "[| p ∈ propn;  q ∈ propn |] ==> H |- p=>q=>p"
        S:  "[| p ∈ propn;  q ∈ propn;  r ∈ propn |]
             ==> H |- (p=>q=>r) => (p=>q) => p=>r"
        DN: "p ∈ propn ==> H |- ((p=>Fls) => Fls) => p"
        MP: "[| H |- p=>q;  H |- p;  p ∈ propn;  q ∈ propn |] ==> H |- q"
      type_intros "propn.intros"

    declare propn.intros [simp]

    subsection {* The semantics *}

    subsubsection {* Semantics of propositional logic. *}

    consts
      is_true_fun :: "[i,i] => i"
    primrec
      "is_true_fun(Fls, t) = 0"
      "is_true_fun(Var(v), t) = (if v ∈ t then 1 else 0)"
      "is_true_fun(p=>q, t) = (if is_true_fun(p,t) = 1 then is_true_fun(q,t) else 1)"

    constdefs
      is_true :: "[i,i] => o"
      "is_true(p,t) == is_true_fun(p,t) = 1"
      -- {* this definition is required since predicates can't be recursive *}

    lemma is_true_Fls [simp]: "is_true(Fls,t) <-> False"
      by (simp add: is_true_def)

    lemma is_true_Var [simp]: "is_true(#v,t) <-> v ∈ t"
      by (simp add: is_true_def)

    lemma is_true_Imp [simp]: "is_true(p=>q,t) <-> (is_true(p,t)-->is_true(q,t))"
      by (simp add: is_true_def)

    subsubsection {* Logical consequence *}

    text {*
      For every valuation, if all elements of @{text H} are true then so
      is @{text p}.
    *}

    constdefs
      logcon :: "[i,i] => o"    (infixl "|=" 50)
      "H |= p == ∀t. (∀q ∈ H. is_true(q,t)) --> is_true(p,t)"

    text {*
      A finite set of hypotheses from @{text t} and the @{text Var}s in
      @{text p}.
    *}

    consts
      hyps :: "[i,i] => i"
    primrec
      "hyps(Fls, t) = 0"
      "hyps(Var(v), t) = (if v ∈ t then {#v} else {#v=>Fls})"
      "hyps(p=>q, t) = hyps(p,t) ∪ hyps(q,t)"

    subsection {* Proof theory of propositional logic *}

    lemma thms_mono: "G ⊆ H ==> thms(G) ⊆ thms(H)"
      apply (unfold thms.defs)
      apply (rule lfp_mono)
        apply (rule thms.bnd_mono)+
      apply (assumption | rule univ_mono basic_monos)+
      done

    lemmas thms_in_pl = thms.dom_subset [THEN subsetD]

    inductive_cases ImpE: "p=>q ∈ propn"

    lemma thms_MP: "[| H |- p=>q;  H |- p |] ==> H |- q"
      -- {* Stronger Modus Ponens rule: no typechecking! *}
      apply (rule thms.MP)
         apply (erule asm_rl thms_in_pl thms_in_pl [THEN ImpE])+
      done

    lemma thms_I: "p ∈ propn ==> H |- p=>p"
      -- {*Rule is called @{text I} for Identity Combinator, not for Introduction. *}
      apply (rule thms.S [THEN thms_MP, THEN thms_MP])
          apply (rule_tac [5] thms.K)
           apply (rule_tac [4] thms.K)
             apply simp_all
      done

    subsubsection {* Weakening, left and right *}

    lemma weaken_left: "[| G ⊆ H;  G|-p |] ==> H|-p"
      -- {* Order of premises is convenient with @{text THEN} *}
      by (erule thms_mono [THEN subsetD])

    lemma weaken_left_cons: "H |- p ==> cons(a,H) |- p"
      by (erule subset_consI [THEN weaken_left])

    lemmas weaken_left_Un1  = Un_upper1 [THEN weaken_left]
    lemmas weaken_left_Un2  = Un_upper2 [THEN weaken_left]

    lemma weaken_right: "[| H |- q;  p ∈ propn |] ==> H |- p=>q"
      by (simp_all add: thms.K [THEN thms_MP] thms_in_pl)

    subsubsection {* The deduction theorem *}

    theorem deduction: "[| cons(p,H) |- q;  p ∈ propn |] ==>  H |- p=>q"
      apply (erule thms.induct)
          apply (blast intro: thms_I thms.H [THEN weaken_right])
         apply (blast intro: thms.K [THEN weaken_right])
        apply (blast intro: thms.S [THEN weaken_right])
       apply (blast intro: thms.DN [THEN weaken_right])
      apply (blast intro: thms.S [THEN thms_MP [THEN thms_MP]])
      done

    subsubsection {* The cut rule *}

    lemma cut: "[| H|-p;  cons(p,H) |- q |] ==>  H |- q"
      apply (rule deduction [THEN thms_MP])
        apply (simp_all add: thms_in_pl)
      done

    lemma thms_FlsE: "[| H |- Fls; p ∈ propn |] ==> H |- p"
      apply (rule thms.DN [THEN thms_MP])
       apply (rule_tac [2] weaken_right)
        apply (simp_all add: propn.intros)
      done

    lemma thms_notE: "[| H |- p=>Fls;  H |- p;  q ∈ propn |] ==> H |- q"
      by (erule thms_MP [THEN thms_FlsE])

    subsubsection {* Soundness of the rules wrt truth-table semantics *}

    theorem soundness: "H |- p ==> H |= p"
      apply (unfold logcon_def)
      apply (erule thms.induct)
          apply auto
      done

    subsection {* Completeness *}

    subsubsection {* Towards the completeness proof *}

    lemma Fls_Imp: "[| H |- p=>Fls; q ∈ propn |] ==> H |- p=>q"
      apply (frule thms_in_pl)
      apply (rule deduction)
       apply (rule weaken_left_cons [THEN thms_notE])
         apply (blast intro: thms.H elim: ImpE)+
      done

    lemma Imp_Fls: "[| H |- p;  H |- q=>Fls |] ==> H |- (p=>q)=>Fls"
      apply (frule thms_in_pl)
      apply (frule thms_in_pl [of concl: "q=>Fls"])
      apply (rule deduction)
       apply (erule weaken_left_cons [THEN thms_MP])
       apply (rule consI1 [THEN thms.H, THEN thms_MP])
        apply (blast intro: weaken_left_cons elim: ImpE)+
      done

    lemma hyps_thms_if:
        "p ∈ propn ==> hyps(p,t) |- (if is_true(p,t) then p else p=>Fls)"
      -- {* Typical example of strengthening the induction statement. *}
      apply simp
      apply (induct_tac p)
        apply (simp_all add: thms_I thms.H)
      apply (safe elim!: Fls_Imp [THEN weaken_left_Un1] Fls_Imp [THEN weaken_left_Un2])
      apply (blast intro: weaken_left_Un1 weaken_left_Un2 weaken_right Imp_Fls)+
      done

    lemma logcon_thms_p: "[| p ∈ propn;  0 |= p |] ==> hyps(p,t) |- p"
      -- {* Key lemma for completeness; yields a set of assumptions satisfying @{text p} *}
      apply (drule hyps_thms_if)
      apply (simp add: logcon_def)
      done

    text {*
      For proving certain theorems in our new propositional logic.
    *}

    lemmas propn_SIs = propn.intros deduction
      and propn_Is = thms_in_pl thms.H thms.H [THEN thms_MP]

    text {*
      The excluded middle in the form of an elimination rule.
    *}

    lemma thms_excluded_middle:
        "[| p ∈ propn;  q ∈ propn |] ==> H |- (p=>q) => ((p=>Fls)=>q) => q"
      apply (rule deduction [THEN deduction])
        apply (rule thms.DN [THEN thms_MP])
         apply (best intro!: propn_SIs intro: propn_Is)+
      done

    lemma thms_excluded_middle_rule:
      "[| cons(p,H) |- q;  cons(p=>Fls,H) |- q;  p ∈ propn |] ==> H |- q"
      -- {* Hard to prove directly because it requires cuts *}
      apply (rule thms_excluded_middle [THEN thms_MP, THEN thms_MP])
         apply (blast intro!: propn_SIs intro: propn_Is)+
      done

    subsubsection {* Completeness -- lemmas for reducing the set of assumptions *}

    text {*
      For the case @{prop "hyps(p,t)-cons(#v,Y) |- p"} we also have @{prop
      "hyps(p,t)-{#v} ⊆ hyps(p, t-{v})"}.
    *}

    lemma hyps_Diff:
        "p ∈ propn ==> hyps(p, t-{v}) ⊆ cons(#v=>Fls, hyps(p,t)-{#v})"
      by (induct_tac p) auto

    text {*
      For the case @{prop "hyps(p,t)-cons(#v => Fls,Y) |- p"} we also have
      @{prop "hyps(p,t)-{#v=>Fls} ⊆ hyps(p, cons(v,t))"}.
    *}

    lemma hyps_cons:
        "p ∈ propn ==> hyps(p, cons(v,t)) ⊆ cons(#v, hyps(p,t)-{#v=>Fls})"
      by (induct_tac p) auto

    text {* Two lemmas for use with @{text weaken_left} *}

    lemma cons_Diff_same: "B-C ⊆ cons(a, B-cons(a,C))"
      by blast

    lemma cons_Diff_subset2: "cons(a, B-{c}) - D ⊆ cons(a, B-cons(c,D))"
      by blast

    text {*
      The set @{term "hyps(p,t)"} is finite, and elements have the form
      @{term "#v"} or @{term "#v=>Fls"}; could probably prove the stronger
      @{prop "hyps(p,t) ∈ Fin(hyps(p,0) ∪ hyps(p,nat))"}.
    *}

    lemma hyps_finite: "p ∈ propn ==> hyps(p,t) ∈ Fin(\<Union>v ∈ nat. {#v, #v=>Fls})"
      by (induct_tac p) auto

    lemmas Diff_weaken_left = Diff_mono [OF _ subset_refl, THEN weaken_left]

    text {*
      Induction on the finite set of assumptions @{term "hyps(p,t0)"}.  We
      may repeatedly subtract assumptions until none are left!
    *}

    lemma completeness_0_lemma [rule_format]:
        "[| p ∈ propn;  0 |= p |] ==> ∀t. hyps(p,t) - hyps(p,t0) |- p"
      apply (frule hyps_finite)
      apply (erule Fin_induct)
       apply (simp add: logcon_thms_p Diff_0)
      txt {* inductive step *}
      apply safe
       txt {* Case @{prop "hyps(p,t)-cons(#v,Y) |- p"} *}
       apply (rule thms_excluded_middle_rule)
         apply (erule_tac [3] propn.intros)
        apply (blast intro: cons_Diff_same [THEN weaken_left])
       apply (blast intro: cons_Diff_subset2 [THEN weaken_left]
         hyps_Diff [THEN Diff_weaken_left])
      txt {* Case @{prop "hyps(p,t)-cons(#v => Fls,Y) |- p"} *}
      apply (rule thms_excluded_middle_rule)
        apply (erule_tac [3] propn.intros)
       apply (blast intro: cons_Diff_subset2 [THEN weaken_left]
         hyps_cons [THEN Diff_weaken_left])
      apply (blast intro: cons_Diff_same [THEN weaken_left])
      done

    subsubsection {* Completeness theorem *}

    lemma completeness_0: "[| p ∈ propn;  0 |= p |] ==> 0 |- p"
      -- {* The base case for completeness *}
      apply (rule Diff_cancel [THEN subst])
      apply (blast intro: completeness_0_lemma)
      done

    lemma logcon_Imp: "[| cons(p,H) |= q |] ==> H |= p=>q"
      -- {* A semantic analogue of the Deduction Theorem *}
      by (simp add: logcon_def)

    lemma completeness [rule_format]:
         "H ∈ Fin(propn) ==> ∀p ∈ propn. H |= p --> H |- p"
      apply (erule Fin_induct)
       apply (safe intro!: completeness_0)
      apply (rule weaken_left_cons [THEN thms_MP])
       apply (blast intro!: logcon_Imp propn.intros)
      apply (blast intro: propn_Is)
      done

    theorem thms_iff: "H ∈ Fin(propn) ==> H |- p <-> H |= p ∧ p ∈ propn"
      by (blast intro: soundness completeness thms_in_pl)

    end

### The datatype of propositions

### The proof system

### The semantics

#### Semantics of propositional logic.

lemma is_true_Fls:

      is_true(Fls, t) <-> False

lemma is_true_Var:

      is_true(#v, t) <-> v ∈ t

lemma is_true_Imp:

      is_true(p => q, t) <-> is_true(p, t) --> is_true(q, t)

#### Logical consequence

### Proof theory of propositional logic

lemma thms_mono:

      G ⊆ H ==> thms(G) ⊆ thms(H)

lemmas thms_in_pl:

      H1 |- c ==> c ∈ propn

lemmas thms_in_pl:

      H1 |- c ==> c ∈ propn

lemmas ImpE:

      [| p => q ∈ propn; [| p ∈ propn; q ∈ propn |] ==> Q |] ==> Q

lemma thms_MP:

      [| H |- p => q; H |- p |] ==> H |- q

lemma thms_I:

      p ∈ propn ==> H |- p => p

#### Weakening, left and right

lemma weaken_left:

      [| G ⊆ H; G |- p |] ==> H |- p

lemma weaken_left_cons:

      H |- p ==> cons(a, H) |- p

lemmas weaken_left_Un1:

      G |- p ==> G ∪ B1 |- p

lemmas weaken_left_Un1:

      G |- p ==> G ∪ B1 |- p

lemmas weaken_left_Un2:

      G |- p ==> A1 ∪ G |- p

lemmas weaken_left_Un2:

      G |- p ==> A1 ∪ G |- p

lemma weaken_right:

      [| H |- q; p ∈ propn |] ==> H |- p => q

#### The deduction theorem

theorem deduction:

      [| cons(p, H) |- q; p ∈ propn |] ==> H |- p => q

#### The cut rule

lemma cut:

      [| H |- p; cons(p, H) |- q |] ==> H |- q

lemma thms_FlsE:

      [| H |- Fls; p ∈ propn |] ==> H |- p

lemma thms_notE:

      [| H |- p => Fls; H |- p; q ∈ propn |] ==> H |- q

#### Soundness of the rules wrt truth-table semantics

theorem soundness:

      H |- p ==> H |= p

### Completeness

#### Towards the completeness proof

lemma Fls_Imp:

      [| H |- p => Fls; q ∈ propn |] ==> H |- p => q

lemma Imp_Fls:

      [| H |- p; H |- q => Fls |] ==> H |- (p => q) => Fls

lemma hyps_thms_if:

      p ∈ propn ==> hyps(p, t) |- (if is_true(p, t) then p else p => Fls)

lemma logcon_thms_p:

      [| p ∈ propn; 0 |= p |] ==> hyps(p, t) |- p

lemmas propn_SIs:

      Fls ∈ propn

      n ∈ nat ==> #n ∈ propn

      [| p ∈ propn; q ∈ propn |] ==> p => q ∈ propn

      [| cons(p, H) |- q; p ∈ propn |] ==> H |- p => q

and propn_Is:

      H1 |- c ==> c ∈ propn

      [| p ∈ H; p ∈ propn |] ==> H |- p

      [| p => q ∈ H; p => q ∈ propn; H |- p |] ==> H |- q

lemmas propn_SIs:

      Fls ∈ propn

      n ∈ nat ==> #n ∈ propn

      [| p ∈ propn; q ∈ propn |] ==> p => q ∈ propn

      [| cons(p, H) |- q; p ∈ propn |] ==> H |- p => q

and propn_Is:

      H1 |- c ==> c ∈ propn

      [| p ∈ H; p ∈ propn |] ==> H |- p

      [| p => q ∈ H; p => q ∈ propn; H |- p |] ==> H |- q

lemma thms_excluded_middle:

      [| p ∈ propn; q ∈ propn |] ==> H |- (p => q) => ((p => Fls) => q) => q

lemma thms_excluded_middle_rule:

      [| cons(p, H) |- q; cons(p => Fls, H) |- q; p ∈ propn |] ==> H |- q

#### Completeness -- lemmas for reducing the set of assumptions

lemma hyps_Diff:

      p ∈ propn ==> hyps(p, t - {v}) ⊆ cons(#v => Fls, hyps(p, t) - {#v})

lemma hyps_cons:

      p ∈ propn ==> hyps(p, cons(v, t)) ⊆ cons(#v, hyps(p, t) - {#v => Fls})

lemma cons_Diff_same:

      B - C ⊆ cons(a, B - cons(a, C))

lemma cons_Diff_subset2:

      cons(a, B - {c}) - D ⊆ cons(a, B - cons(c, D))

lemma hyps_finite:

      p ∈ propn ==> hyps(p, t) ∈ Fin(\<Union>v∈nat. {#v, #v => Fls})

lemmas Diff_weaken_left:

      [| A1 ⊆ C1; A1 - B1 |- p |] ==> C1 - B1 |- p

lemmas Diff_weaken_left:

      [| A1 ⊆ C1; A1 - B1 |- p |] ==> C1 - B1 |- p

lemma completeness_0_lemma:

      [| p ∈ propn; 0 |= p |] ==> hyps(p, t) - hyps(p, t0.0) |- p

#### Completeness theorem

lemma completeness_0:

      [| p ∈ propn; 0 |= p |] ==> 0 |- p

lemma logcon_Imp:

      cons(p, H) |= q ==> H |= p => q

lemma completeness:

      [| H ∈ Fin(propn); p ∈ propn; H |= p |] ==> H |- p

theorem thms_iff:

      H ∈ Fin(propn) ==> H |- p <-> H |= p ∧ p ∈ propn