Theory PropLog

theory PropLog
imports Main
(*  Title:      HOL/Induct/PropLog.thy
Author: Tobias Nipkow
Copyright 1994 TU Muenchen & 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 ∈

subsection {* The datatype of propositions *}

datatype 'a pl =
false |
var 'a ("#_" [1000]) |
imp "'a pl" "'a pl" (infixr "->" 90)

subsection {* The proof system *}

inductive thms :: "['a pl set, 'a pl] => bool" (infixl "|-" 50)
for H :: "'a pl set"
H: "p∈H ==> H |- p"
| K: "H |- p->q->p"
| S: "H |- (p->q->r) -> (p->q) -> p->r"
| DN: "H |- ((p->false) -> false) -> p"
| MP: "[| H |- p->q; H |- p |] ==> H |- q"

subsection {* The semantics *}

subsubsection {* Semantics of propositional logic. *}

primrec eval :: "['a set, 'a pl] => bool" ("_[[_]]" [100,0] 100)
"tt[[false]] = False"
| "tt[[#v]] = (v ∈ tt)"
| eval_imp: "tt[[p->q]] = (tt[[p]] --> tt[[q]])"

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

primrec hyps :: "['a pl, 'a set] => 'a pl set"
"hyps false tt = {}"
| "hyps (#v) tt = {if v ∈ tt then #v else #v->false}"
| "hyps (p->q) tt = hyps p tt Un hyps q tt"

subsubsection {* Logical consequence *}

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

definition sat :: "['a pl set, 'a pl] => bool" (infixl "|=" 50)
where "H |= p = (∀tt. (∀q∈H. tt[[q]]) --> tt[[p]])"

subsection {* Proof theory of propositional logic *}

lemma thms_mono: "G<=H ==> thms(G) <= thms(H)"
apply (rule predicate1I)
apply (erule thms.induct)
apply (auto intro: thms.intros)

lemma thms_I: "H |- p->p"
-- {*Called @{text I} for Identity Combinator, not for Introduction. *}
by (best intro: thms.K thms.S thms.MP)

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 predicate1D])

lemma weaken_left_insert: "G |- p ==> insert a G |- p"
by (rule weaken_left) (rule subset_insertI)

lemma weaken_left_Un1: "G |- p ==> G ∪ B |- p"
by (rule weaken_left) (rule Un_upper1)

lemma weaken_left_Un2: "G |- p ==> A ∪ G |- p"
by (rule weaken_left) (rule Un_upper2)

lemma weaken_right: "H |- q ==> H |- p->q"
by (fast intro: thms.K thms.MP)

subsubsection {* The deduction theorem *}

theorem deduction: "insert p H |- q ==> H |- p->q"
apply (induct set: thms)
apply (fast intro: thms_I thms.H thms.K thms.S thms.DN
thms.S [THEN thms.MP, THEN thms.MP] weaken_right)+

subsubsection {* The cut rule *}

lemma cut: "insert p H |- q ==> H |- p ==> H |- q"
by (rule thms.MP) (rule deduction)

lemma thms_falseE: "H |- false ==> H |- q"
by (rule thms.MP, rule thms.DN) (rule weaken_right)

lemma thms_notE: "H |- p -> false ==> H |- p ==> H |- q"
by (rule thms_falseE) (rule thms.MP)

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

theorem soundness: "H |- p ==> H |= p"
by (induct set: thms) (auto simp: sat_def)

subsection {* Completeness *}

subsubsection {* Towards the completeness proof *}

lemma false_imp: "H |- p->false ==> H |- p->q"
apply (rule deduction)
apply (metis H insert_iff weaken_left_insert thms_notE)

lemma imp_false:
"[| H |- p; H |- q->false |] ==> H |- (p->q)->false"
apply (rule deduction)
apply (metis H MP insert_iff weaken_left_insert)

lemma hyps_thms_if: "hyps p tt |- (if tt[[p]] then p else p->false)"
-- {* Typical example of strengthening the induction statement. *}
apply simp
apply (induct p)
apply (simp_all add: thms_I thms.H)
apply (blast intro: weaken_left_Un1 weaken_left_Un2 weaken_right
imp_false false_imp)

lemma sat_thms_p: "{} |= p ==> hyps p tt |- p"
-- {* Key lemma for completeness; yields a set of assumptions
satisfying @{text p} *}

apply (unfold sat_def)
apply (drule spec, erule mp [THEN if_P, THEN subst],
rule_tac [2] hyps_thms_if, simp)

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

declare deduction [intro!]
declare thms.H [THEN thms.MP, intro]

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

lemma thms_excluded_middle: "H |- (p->q) -> ((p->false)->q) -> q"
apply (rule deduction [THEN deduction])
apply (rule thms.DN [THEN thms.MP], best intro: H)

lemma thms_excluded_middle_rule:
"[| insert p H |- q; insert (p->false) H |- q |] ==> H |- q"
-- {* Hard to prove directly because it requires cuts *}
by (rule thms_excluded_middle [THEN thms.MP, THEN thms.MP], auto)

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

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

lemma hyps_Diff: "hyps p (t-{v}) <= insert (#v->false) ((hyps p t)-{#v})"
by (induct p) auto

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

lemma hyps_insert: "hyps p (insert v t) <= insert (#v) (hyps p t-{#v->false})"
by (induct p) auto

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

lemma insert_Diff_same: "B-C <= insert a (B-insert a C)"
by fast

lemma insert_Diff_subset2: "insert a (B-{c}) - D <= insert a (B-insert c D)"
by fast

text {*
The set @{term "hyps p t"} is finite, and elements have the form
@{term "#v"} or @{term "#v->Fls"}.

lemma hyps_finite: "finite(hyps p t)"
by (induct p) auto

lemma hyps_subset: "hyps p t <= (UN v. {#v, #v->false})"
by (induct p) auto

lemma Diff_weaken_left: "A ⊆ C ==> A - B |- p ==> C - B |- p"
by (rule Diff_mono [OF _ subset_refl, THEN weaken_left])

subsubsection {* Completeness theorem *}

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:
"{} |= p ==> ∀t. hyps p t - hyps p t0 |- p"
apply (rule hyps_subset [THEN hyps_finite [THEN finite_subset_induct]])
apply (simp add: sat_thms_p, safe)
txt{*Case @{text"hyps p t-insert(#v,Y) |- p"} *}
apply (iprover intro: thms_excluded_middle_rule
insert_Diff_same [THEN weaken_left]
insert_Diff_subset2 [THEN weaken_left]
hyps_Diff [THEN Diff_weaken_left])
txt{*Case @{text"hyps p t-insert(#v -> false,Y) |- p"} *}
apply (iprover intro: thms_excluded_middle_rule
insert_Diff_same [THEN weaken_left]
insert_Diff_subset2 [THEN weaken_left]
hyps_insert [THEN Diff_weaken_left])

text{*The base case for completeness*}
lemma completeness_0: "{} |= p ==> {} |- p"
apply (rule Diff_cancel [THEN subst])
apply (erule completeness_0_lemma [THEN spec])

text{*A semantic analogue of the Deduction Theorem*}
lemma sat_imp: "insert p H |= q ==> H |= p->q"
by (auto simp: sat_def)

theorem completeness: "finite H ==> H |= p ==> H |- p"
apply (induct arbitrary: p rule: finite_induct)
apply (blast intro: completeness_0)
apply (iprover intro: sat_imp thms.H insertI1 weaken_left_insert [THEN thms.MP])

theorem syntax_iff_semantics: "finite H ==> (H |- p) = (H |= p)"
by (blast intro: soundness completeness)