901 lines
37 KiB
Text
901 lines
37 KiB
Text
import macros
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import tactic
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universe U ≥ 1
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definition TypeU := (Type U)
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-- create default rewrite rule set
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(* mk_rewrite_rule_set() *)
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variable Bool : Type
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-- Heterogeneous equality
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variable heq {A B : (Type U)} (a : A) (b : B) : Bool
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infixl 50 == : heq
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-- Reflexivity for heterogeneous equality
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axiom hrefl {A : (Type U)} (a : A) : a == a
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-- Homogeneous equality
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definition eq {A : (Type U)} (a b : A) := a == b
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infix 50 = : eq
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theorem refl {A : (Type U)} (a : A) : a = a
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:= hrefl a
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theorem heq_eq {A : (Type U)} (a b : A) : (a == b) = (a = b)
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:= refl (a == b)
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definition true : Bool
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:= (λ x : Bool, x) = (λ x : Bool, x)
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theorem trivial : true
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:= refl (λ x : Bool, x)
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set_opaque true true
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definition false : Bool
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:= ∀ x : Bool, x
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set_opaque false true
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alias ⊤ : true
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alias ⊥ : false
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definition not (a : Bool) := a → false
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notation 40 ¬ _ : not
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definition or (a b : Bool) := ¬ a → b
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infixr 30 || : or
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infixr 30 \/ : or
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infixr 30 ∨ : or
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definition and (a b : Bool) := ¬ (a → ¬ b)
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infixr 35 && : and
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infixr 35 /\ : and
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infixr 35 ∧ : and
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definition implies (a b : Bool) := a → b
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definition neq {A : (Type U)} (a b : A) := ¬ (a = b)
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infix 50 ≠ : neq
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theorem a_neq_a_elim {A : (Type U)} {a : A} (H : a ≠ a) : false
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:= H (refl a)
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definition iff (a b : Bool) := a = b
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infixr 25 <-> : iff
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infixr 25 ↔ : iff
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theorem em (a : Bool) : a ∨ ¬ a
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:= assume Hna : ¬ a, Hna
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theorem not_intro {a : Bool} (H : a → false) : ¬ a
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:= H
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theorem absurd {a : Bool} (H1 : a) (H2 : ¬ a) : false
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:= H2 H1
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-- The Lean parser has special treatment for the constant exists.
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-- It allows us to write
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-- exists x y : A, P x y and ∃ x y : A, P x y
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-- as syntax sugar for
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-- exists A (fun x : A, exists A (fun y : A, P x y))
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-- That is, it treats the exists as an extra binder such as fun and forall.
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-- It also provides an alias (Exists) that should be used when we
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-- want to treat exists as a constant.
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definition Exists (A : (Type U)) (P : A → Bool)
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:= ¬ (∀ x, ¬ (P x))
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axiom case (P : Bool → Bool) (H1 : P true) (H2 : P false) (a : Bool) : P a
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theorem false_elim (a : Bool) (H : false) : a
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:= case (λ x, x) trivial H a
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theorem mt {a b : Bool} (H1 : a → b) (H2 : ¬ b) : ¬ a
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:= assume Ha : a, absurd (H1 Ha) H2
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theorem contrapos {a b : Bool} (H : a → b) : ¬ b → ¬ a
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:= assume Hnb : ¬ b, mt H Hnb
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theorem absurd_elim {a : Bool} (b : Bool) (H1 : a) (H2 : ¬ a) : b
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:= false_elim b (absurd H1 H2)
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-- Recall that or is defined as ¬ a → b
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theorem or_introl {a : Bool} (H : a) (b : Bool) : a ∨ b
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:= assume H1 : ¬ a, absurd_elim b H H1
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theorem or_intror {b : Bool} (a : Bool) (H : b) : a ∨ b
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:= assume H1 : ¬ a, H
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theorem boolcomplete (a : Bool) : a = true ∨ a = false
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:= case (λ x, x = true ∨ x = false)
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(or_introl (refl true) (true = false))
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(or_intror (false = true) (refl false))
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a
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theorem boolcomplete_swapped (a : Bool) : a = false ∨ a = true
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:= case (λ x, x = false ∨ x = true)
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(or_intror (true = false) (refl true))
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(or_introl (refl false) (false = true))
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a
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theorem resolve1 {a b : Bool} (H1 : a ∨ b) (H2 : ¬ a) : b
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:= H1 H2
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axiom subst {A : (Type U)} {a b : A} {P : A → Bool} (H1 : P a) (H2 : a = b) : P b
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-- Alias for subst where we provide P explicitly, but keep A,a,b implicit
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theorem substp {A : (Type U)} {a b : A} (P : A → Bool) (H1 : P a) (H2 : a = b) : P b
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:= subst H1 H2
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theorem symm {A : (Type U)} {a b : A} (H : a = b) : b = a
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:= subst (refl a) H
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theorem trans {A : (Type U)} {a b c : A} (H1 : a = b) (H2 : b = c) : a = c
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:= subst H1 H2
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theorem congr1 {A B : (Type U)} {f g : A → B} (a : A) (H : f = g) : f a = g a
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:= substp (fun h : A → B, f a = h a) (refl (f a)) H
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theorem congr2 {A B : (Type U)} {a b : A} (f : A → B) (H : a = b) : f a = f b
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:= substp (fun x : A, f a = f x) (refl (f a)) H
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theorem congr {A B : (Type U)} {f g : A → B} {a b : A} (H1 : f = g) (H2 : a = b) : f a = g b
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:= subst (congr2 f H2) (congr1 b H1)
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theorem true_ne_false : ¬ true = false
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:= assume H : true = false,
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subst trivial H
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theorem absurd_not_true (H : ¬ true) : false
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:= absurd trivial H
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theorem not_false_trivial : ¬ false
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:= assume H : false, H
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-- "equality modus pones"
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theorem eqmp {a b : Bool} (H1 : a = b) (H2 : a) : b
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:= subst H2 H1
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infixl 100 <| : eqmp
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infixl 100 ◂ : eqmp
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theorem eqmpr {a b : Bool} (H1 : a = b) (H2 : b) : a
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:= (symm H1) ◂ H2
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theorem imp_trans {a b c : Bool} (H1 : a → b) (H2 : b → c) : a → c
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:= assume Ha, H2 (H1 Ha)
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theorem imp_eq_trans {a b c : Bool} (H1 : a → b) (H2 : b = c) : a → c
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:= assume Ha, H2 ◂ (H1 Ha)
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theorem eq_imp_trans {a b c : Bool} (H1 : a = b) (H2 : b → c) : a → c
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:= assume Ha, H2 (H1 ◂ Ha)
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theorem to_eq {A : TypeU} {a b : A} (H : a == b) : a = b
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:= (heq_eq a b) ◂ H
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theorem to_heq {A : TypeU} {a b : A} (H : a = b) : a == b
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:= (symm (heq_eq a b)) ◂ H
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theorem iff_eliml {a b : Bool} (H : a ↔ b) : a → b
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:= (λ Ha : a, eqmp H Ha)
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theorem iff_elimr {a b : Bool} (H : a ↔ b) : b → a
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:= (λ Hb : b, eqmpr H Hb)
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theorem ne_symm {A : TypeU} {a b : A} (H : a ≠ b) : b ≠ a
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:= assume H1 : b = a, H (symm H1)
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theorem eq_ne_trans {A : TypeU} {a b c : A} (H1 : a = b) (H2 : b ≠ c) : a ≠ c
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:= subst H2 (symm H1)
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theorem ne_eq_trans {A : TypeU} {a b c : A} (H1 : a ≠ b) (H2 : b = c) : a ≠ c
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:= subst H1 H2
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theorem eqt_elim {a : Bool} (H : a = true) : a
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:= (symm H) ◂ trivial
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theorem eqf_elim {a : Bool} (H : a = false) : ¬ a
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:= not_intro (λ Ha : a, H ◂ Ha)
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theorem heqt_elim {a : Bool} (H : a == true) : a
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:= eqt_elim (to_eq H)
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theorem not_true : (¬ true) = false
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:= let aux : ¬ (¬ true) = true
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:= assume H : (¬ true) = true,
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absurd_not_true (subst trivial (symm H))
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in resolve1 (boolcomplete (¬ true)) aux
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theorem not_false : (¬ false) = true
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:= let aux : ¬ (¬ false) = false
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:= assume H : (¬ false) = false,
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subst not_false_trivial H
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in resolve1 (boolcomplete_swapped (¬ false)) aux
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add_rewrite not_true not_false
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theorem not_not_eq (a : Bool) : (¬ ¬ a) = a
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:= case (λ x, (¬ ¬ x) = x)
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(calc (¬ ¬ true) = (¬ false) : { not_true }
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... = true : not_false)
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(calc (¬ ¬ false) = (¬ true) : { not_false }
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... = false : not_true)
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a
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add_rewrite not_not_eq
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theorem not_neq {A : TypeU} (a b : A) : ¬ (a ≠ b) ↔ a = b
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:= not_not_eq (a = b)
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add_rewrite not_neq
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theorem not_neq_elim {A : TypeU} {a b : A} (H : ¬ (a ≠ b)) : a = b
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:= (not_neq a b) ◂ H
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theorem not_not_elim {a : Bool} (H : ¬ ¬ a) : a
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:= (not_not_eq a) ◂ H
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theorem not_imp_eliml {a b : Bool} (Hnab : ¬ (a → b)) : a
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:= not_not_elim
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(have ¬ ¬ a :
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assume Hna : ¬ a, absurd (assume Ha : a, absurd_elim b Ha Hna)
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Hnab)
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theorem not_imp_elimr {a b : Bool} (H : ¬ (a → b)) : ¬ b
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:= assume Hb : b, absurd (assume Ha : a, Hb)
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H
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-- Recall that and is defined as ¬ (a → ¬ b)
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theorem and_intro {a b : Bool} (H1 : a) (H2 : b) : a ∧ b
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:= assume H : a → ¬ b, absurd H2 (H H1)
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theorem and_eliml {a b : Bool} (H : a ∧ b) : a
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:= not_imp_eliml H
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theorem and_elimr {a b : Bool} (H : a ∧ b) : b
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:= not_not_elim (not_imp_elimr H)
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theorem or_elim {a b c : Bool} (H1 : a ∨ b) (H2 : a → c) (H3 : b → c) : c
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:= not_not_elim
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(assume H : ¬ c,
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absurd (have c : H3 (have b : resolve1 H1 (have ¬ a : (mt (assume Ha : a, H2 Ha) H))))
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H)
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theorem refute {a : Bool} (H : ¬ a → false) : a
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:= or_elim (em a) (λ H1 : a, H1) (λ H1 : ¬ a, false_elim a (H H1))
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theorem boolext {a b : Bool} (Hab : a → b) (Hba : b → a) : a = b
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:= or_elim (boolcomplete a)
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(λ Hat : a = true, or_elim (boolcomplete b)
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(λ Hbt : b = true, trans Hat (symm Hbt))
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(λ Hbf : b = false, false_elim (a = b) (subst (Hab (eqt_elim Hat)) Hbf)))
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(λ Haf : a = false, or_elim (boolcomplete b)
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(λ Hbt : b = true, false_elim (a = b) (subst (Hba (eqt_elim Hbt)) Haf))
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(λ Hbf : b = false, trans Haf (symm Hbf)))
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-- Another name for boolext
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theorem iff_intro {a b : Bool} (Hab : a → b) (Hba : b → a) : a ↔ b
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:= boolext Hab Hba
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theorem eqt_intro {a : Bool} (H : a) : a = true
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:= boolext (assume H1 : a, trivial)
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(assume H2 : true, H)
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theorem eqf_intro {a : Bool} (H : ¬ a) : a = false
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:= boolext (assume H1 : a, absurd H1 H)
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(assume H2 : false, false_elim a H2)
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theorem a_neq_a {A : (Type U)} (a : A) : (a ≠ a) ↔ false
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:= boolext (assume H, a_neq_a_elim H)
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(assume H, false_elim (a ≠ a) H)
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theorem eq_id {A : (Type U)} (a : A) : (a = a) ↔ true
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:= eqt_intro (refl a)
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theorem iff_id (a : Bool) : (a ↔ a) ↔ true
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:= eqt_intro (refl a)
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theorem neq_elim {A : (Type U)} {a b : A} (H : a ≠ b) : a = b ↔ false
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:= eqf_intro H
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theorem neq_to_not_eq {A : (Type U)} {a b : A} : a ≠ b ↔ ¬ a = b
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:= refl (a ≠ b)
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add_rewrite eq_id iff_id neq_to_not_eq
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-- Remark: ordered rewriting + assoc + comm + left_comm sorts a term lexicographically
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theorem left_comm {A : (Type U)} {R : A -> A -> A} (comm : ∀ x y, R x y = R y x) (assoc : ∀ x y z, R (R x y) z = R x (R y z)) :
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∀ x y z, R x (R y z) = R y (R x z)
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:= take x y z, calc R x (R y z) = R (R x y) z : symm (assoc x y z)
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... = R (R y x) z : { comm x y }
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... = R y (R x z) : assoc y x z
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theorem or_comm (a b : Bool) : (a ∨ b) = (b ∨ a)
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:= boolext (assume H, or_elim H (λ H1, or_intror b H1) (λ H2, or_introl H2 a))
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(assume H, or_elim H (λ H1, or_intror a H1) (λ H2, or_introl H2 b))
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theorem or_assoc (a b c : Bool) : (a ∨ b) ∨ c ↔ a ∨ (b ∨ c)
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:= boolext (assume H : (a ∨ b) ∨ c,
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or_elim H (λ H1 : a ∨ b, or_elim H1 (λ Ha : a, or_introl Ha (b ∨ c))
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(λ Hb : b, or_intror a (or_introl Hb c)))
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(λ Hc : c, or_intror a (or_intror b Hc)))
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(assume H : a ∨ (b ∨ c),
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or_elim H (λ Ha : a, (or_introl (or_introl Ha b) c))
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(λ H1 : b ∨ c, or_elim H1 (λ Hb : b, or_introl (or_intror a Hb) c)
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(λ Hc : c, or_intror (a ∨ b) Hc)))
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theorem or_id (a : Bool) : a ∨ a ↔ a
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:= boolext (assume H, or_elim H (λ H1, H1) (λ H2, H2))
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(assume H, or_introl H a)
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theorem or_falsel (a : Bool) : a ∨ false ↔ a
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:= boolext (assume H, or_elim H (λ H1, H1) (λ H2, false_elim a H2))
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(assume H, or_introl H false)
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theorem or_falser (a : Bool) : false ∨ a ↔ a
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:= trans (or_comm false a) (or_falsel a)
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theorem or_truel (a : Bool) : true ∨ a ↔ true
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:= boolext (assume H : true ∨ a, trivial)
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(assume H : true, or_introl trivial a)
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theorem or_truer (a : Bool) : a ∨ true ↔ true
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:= trans (or_comm a true) (or_truel a)
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theorem or_tauto (a : Bool) : a ∨ ¬ a ↔ true
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:= eqt_intro (em a)
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theorem or_left_comm (a b c : Bool) : a ∨ (b ∨ c) ↔ b ∨ (a ∨ c)
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:= left_comm or_comm or_assoc a b c
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add_rewrite or_comm or_assoc or_id or_falsel or_falser or_truel or_truer or_tauto or_left_comm
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theorem resolve2 {a b : Bool} (H1 : a ∨ b) (H2 : ¬ b) : a
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:= resolve1 ((or_comm a b) ◂ H1) H2
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theorem and_comm (a b : Bool) : a ∧ b ↔ b ∧ a
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:= boolext (assume H, and_intro (and_elimr H) (and_eliml H))
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(assume H, and_intro (and_elimr H) (and_eliml H))
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theorem and_id (a : Bool) : a ∧ a ↔ a
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:= boolext (assume H, and_eliml H)
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(assume H, and_intro H H)
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theorem and_assoc (a b c : Bool) : (a ∧ b) ∧ c ↔ a ∧ (b ∧ c)
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:= boolext (assume H, and_intro (and_eliml (and_eliml H)) (and_intro (and_elimr (and_eliml H)) (and_elimr H)))
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(assume H, and_intro (and_intro (and_eliml H) (and_eliml (and_elimr H))) (and_elimr (and_elimr H)))
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theorem and_truer (a : Bool) : a ∧ true ↔ a
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:= boolext (assume H : a ∧ true, and_eliml H)
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(assume H : a, and_intro H trivial)
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theorem and_truel (a : Bool) : true ∧ a ↔ a
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:= trans (and_comm true a) (and_truer a)
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theorem and_falsel (a : Bool) : a ∧ false ↔ false
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:= boolext (assume H, and_elimr H)
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(assume H, false_elim (a ∧ false) H)
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theorem and_falser (a : Bool) : false ∧ a ↔ false
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:= trans (and_comm false a) (and_falsel a)
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theorem and_absurd (a : Bool) : a ∧ ¬ a ↔ false
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:= boolext (assume H, absurd (and_eliml H) (and_elimr H))
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(assume H, false_elim (a ∧ ¬ a) H)
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theorem and_left_comm (a b c : Bool) : a ∧ (b ∧ c) ↔ b ∧ (a ∧ c)
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:= left_comm and_comm and_assoc a b c
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add_rewrite and_comm and_assoc and_id and_falsel and_falser and_truel and_truer and_absurd and_left_comm
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theorem imp_truer (a : Bool) : (a → true) ↔ true
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:= boolext (assume H, trivial)
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(assume H Ha, trivial)
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theorem imp_truel (a : Bool) : (true → a) ↔ a
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:= boolext (assume H : true → a, H trivial)
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(assume Ha H, Ha)
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theorem imp_falser (a : Bool) : (a → false) ↔ ¬ a
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:= refl _
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theorem imp_falsel (a : Bool) : (false → a) ↔ true
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:= boolext (assume H, trivial)
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(assume H Hf, false_elim a Hf)
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||
theorem imp_id (a : Bool) : (a → a) ↔ true
|
||
:= eqt_intro (λ H : a, H)
|
||
|
||
add_rewrite imp_truer imp_truel imp_falser imp_falsel imp_id
|
||
|
||
theorem imp_or (a b : Bool) : (a → b) ↔ ¬ a ∨ b
|
||
:= boolext
|
||
(assume H : a → b,
|
||
(or_elim (em a)
|
||
(λ Ha : a, or_intror (¬ a) (H Ha))
|
||
(λ Hna : ¬ a, or_introl Hna b)))
|
||
(assume H : ¬ a ∨ b,
|
||
assume Ha : a,
|
||
resolve1 H ((symm (not_not_eq a)) ◂ Ha))
|
||
|
||
theorem not_congr {a b : Bool} (H : a ↔ b) : ¬ a ↔ ¬ b
|
||
:= congr2 not H
|
||
|
||
-- Recall that exists is defined as ¬ ∀ x : A, ¬ P x
|
||
theorem exists_elim {A : TypeU} {P : A → Bool} {B : Bool} (H1 : Exists A P) (H2 : ∀ (a : A) (H : P a), B) : B
|
||
:= refute (λ R : ¬ B,
|
||
absurd (take a : A, mt (assume H : P a, H2 a H) R)
|
||
H1)
|
||
|
||
theorem exists_intro {A : TypeU} {P : A → Bool} (a : A) (H : P a) : Exists A P
|
||
:= assume H1 : (∀ x : A, ¬ P x),
|
||
absurd H (H1 a)
|
||
|
||
theorem not_exists (A : (Type U)) (P : A → Bool) : ¬ (∃ x : A, P x) ↔ (∀ x : A, ¬ P x)
|
||
:= calc (¬ ∃ x : A, P x) = ¬ ¬ ∀ x : A, ¬ P x : refl (¬ ∃ x : A, P x)
|
||
... = ∀ x : A, ¬ P x : not_not_eq (∀ x : A, ¬ P x)
|
||
|
||
theorem not_exists_elim {A : (Type U)} {P : A → Bool} (H : ¬ ∃ x : A, P x) : ∀ x : A, ¬ P x
|
||
:= (not_exists A P) ◂ H
|
||
|
||
theorem exists_unfold1 {A : TypeU} {P : A → Bool} (a : A) (H : ∃ x : A, P x) : P a ∨ (∃ x : A, x ≠ a ∧ P x)
|
||
:= exists_elim H
|
||
(λ (w : A) (H1 : P w),
|
||
or_elim (em (w = a))
|
||
(λ Heq : w = a, or_introl (subst H1 Heq) (∃ x : A, x ≠ a ∧ P x))
|
||
(λ Hne : w ≠ a, or_intror (P a) (exists_intro w (and_intro Hne H1))))
|
||
|
||
theorem exists_unfold2 {A : TypeU} {P : A → Bool} (a : A) (H : P a ∨ (∃ x : A, x ≠ a ∧ P x)) : ∃ x : A, P x
|
||
:= or_elim H
|
||
(λ H1 : P a, exists_intro a H1)
|
||
(λ H2 : (∃ x : A, x ≠ a ∧ P x),
|
||
exists_elim H2
|
||
(λ (w : A) (Hw : w ≠ a ∧ P w),
|
||
exists_intro w (and_elimr Hw)))
|
||
|
||
theorem exists_unfold {A : TypeU} (P : A → Bool) (a : A) : (∃ x : A, P x) ↔ (P a ∨ (∃ x : A, x ≠ a ∧ P x))
|
||
:= boolext (assume H : (∃ x : A, P x), exists_unfold1 a H)
|
||
(assume H : (P a ∨ (∃ x : A, x ≠ a ∧ P x)), exists_unfold2 a H)
|
||
|
||
definition inhabited (A : (Type U))
|
||
:= ∃ x : A, true
|
||
|
||
-- If we have an element of type A, then A is inhabited
|
||
theorem inhabited_intro {A : TypeU} (a : A) : inhabited A
|
||
:= assume H : (∀ x, ¬ true), absurd_not_true (H a)
|
||
|
||
theorem inhabited_elim {A : TypeU} (H1 : inhabited A) {B : Bool} (H2 : A → B) : B
|
||
:= obtain (w : A) (Hw : true), from H1,
|
||
H2 w
|
||
|
||
theorem inhabited_ex_intro {A : TypeU} {P : A → Bool} (H : ∃ x, P x) : inhabited A
|
||
:= obtain (w : A) (Hw : P w), from H,
|
||
exists_intro w trivial
|
||
|
||
-- If a function space is non-empty, then for every 'a' in the domain, the range (B a) is not empty
|
||
theorem inhabited_range {A : TypeU} {B : A → TypeU} (H : inhabited (∀ x, B x)) (a : A) : inhabited (B a)
|
||
:= refute (λ N : ¬ inhabited (B a),
|
||
let s1 : ¬ ∃ x : B a, true := N,
|
||
s2 : ∀ x : B a, false := take x : B a, absurd_not_true (not_exists_elim s1 x),
|
||
s3 : ∃ y : (∀ x, B x), true := H
|
||
in obtain (w : (∀ x, B x)) (Hw : true), from s3,
|
||
let s4 : B a := w a
|
||
in s2 s4)
|
||
|
||
theorem exists_rem {A : TypeU} (H : inhabited A) (p : Bool) : (∃ x : A, p) ↔ p
|
||
:= iff_intro
|
||
(assume Hl : (∃ x : A, p),
|
||
obtain (w : A) (Hw : p), from Hl,
|
||
Hw)
|
||
(assume Hr : p,
|
||
inhabited_elim H (λ w, exists_intro w Hr))
|
||
|
||
theorem forall_rem {A : TypeU} (H : inhabited A) (p : Bool) : (∀ x : A, p) ↔ p
|
||
:= iff_intro
|
||
(assume Hl : (∀ x : A, p),
|
||
inhabited_elim H (λ w, Hl w))
|
||
(assume Hr : p,
|
||
take x, Hr)
|
||
|
||
-- Congruence theorems for contextual simplification
|
||
|
||
-- Simplify a → b, by first simplifying a to c using the fact that ¬ b is true, and then
|
||
-- b to d using the fact that c is true
|
||
theorem imp_congrr {a b c d : Bool} (H_ac : ∀ (H_nb : ¬ b), a = c) (H_bd : ∀ (H_c : c), b = d) : (a → b) = (c → d)
|
||
:= or_elim (em b)
|
||
(λ H_b : b,
|
||
or_elim (em c)
|
||
(λ H_c : c,
|
||
calc (a → b) = (a → true) : { eqt_intro H_b }
|
||
... = true : imp_truer a
|
||
... = (c → true) : symm (imp_truer c)
|
||
... = (c → b) : { symm (eqt_intro H_b) }
|
||
... = (c → d) : { H_bd H_c })
|
||
(λ H_nc : ¬ c,
|
||
calc (a → b) = (a → true) : { eqt_intro H_b }
|
||
... = true : imp_truer a
|
||
... = (false → d) : symm (imp_falsel d)
|
||
... = (c → d) : { symm (eqf_intro H_nc) }))
|
||
(λ H_nb : ¬ b,
|
||
or_elim (em c)
|
||
(λ H_c : c,
|
||
calc (a → b) = (c → b) : { H_ac H_nb }
|
||
... = (c → d) : { H_bd H_c })
|
||
(λ H_nc : ¬ c,
|
||
calc (a → b) = (c → b) : { H_ac H_nb }
|
||
... = (false → b) : { eqf_intro H_nc }
|
||
... = true : imp_falsel b
|
||
... = (false → d) : symm (imp_falsel d)
|
||
... = (c → d) : { symm (eqf_intro H_nc) }))
|
||
|
||
|
||
-- Simplify a → b, by first simplifying b to d using the fact that a is true, and then
|
||
-- b to d using the fact that ¬ d is true.
|
||
-- This kind of congruence seems to be useful in very rare cases.
|
||
theorem imp_congrl {a b c d : Bool} (H_bd : ∀ (H_a : a), b = d) (H_ac : ∀ (H_nd : ¬ d), a = c) : (a → b) = (c → d)
|
||
:= or_elim (em a)
|
||
(λ H_a : a,
|
||
or_elim (em d)
|
||
(λ H_d : d,
|
||
calc (a → b) = (a → d) : { H_bd H_a }
|
||
... = (a → true) : { eqt_intro H_d }
|
||
... = true : imp_truer a
|
||
... = (c → true) : symm (imp_truer c)
|
||
... = (c → d) : { symm (eqt_intro H_d) })
|
||
(λ H_nd : ¬ d,
|
||
calc (a → b) = (c → b) : { H_ac H_nd }
|
||
... = (c → d) : { H_bd H_a }))
|
||
(λ H_na : ¬ a,
|
||
or_elim (em d)
|
||
(λ H_d : d,
|
||
calc (a → b) = (false → b) : { eqf_intro H_na }
|
||
... = true : imp_falsel b
|
||
... = (c → true) : symm (imp_truer c)
|
||
... = (c → d) : { symm (eqt_intro H_d) })
|
||
(λ H_nd : ¬ d,
|
||
calc (a → b) = (false → b) : { eqf_intro H_na }
|
||
... = true : imp_falsel b
|
||
... = (false → d) : symm (imp_falsel d)
|
||
... = (a → d) : { symm (eqf_intro H_na) }
|
||
... = (c → d) : { H_ac H_nd }))
|
||
|
||
-- (Common case) simplify a to c, and then b to d using the fact that c is true
|
||
theorem imp_congr {a b c d : Bool} (H_ac : a = c) (H_bd : ∀ (H_c : c), b = d) : (a → b) = (c → d)
|
||
:= imp_congrr (λ H, H_ac) H_bd
|
||
|
||
-- In the following theorems we are using the fact that a ∨ b is defined as ¬ a → b
|
||
theorem or_congrr {a b c d : Bool} (H_ac : ∀ (H_nb : ¬ b), a = c) (H_bd : ∀ (H_nc : ¬ c), b = d) : a ∨ b ↔ c ∨ d
|
||
:= imp_congrr (λ H_nb : ¬ b, congr2 not (H_ac H_nb)) H_bd
|
||
theorem or_congrl {a b c d : Bool} (H_bd : ∀ (H_na : ¬ a), b = d) (H_ac : ∀ (H_nd : ¬ d), a = c) : a ∨ b ↔ c ∨ d
|
||
:= imp_congrl H_bd (λ H_nd : ¬ d, congr2 not (H_ac H_nd))
|
||
-- (Common case) simplify a to c, and then b to d using the fact that ¬ c is true
|
||
theorem or_congr {a b c d : Bool} (H_ac : a = c) (H_bd : ∀ (H_nc : ¬ c), b = d) : a ∨ b ↔ c ∨ d
|
||
:= or_congrr (λ H, H_ac) H_bd
|
||
|
||
-- In the following theorems we are using the fact hat a ∧ b is defined as ¬ (a → ¬ b)
|
||
theorem and_congrr {a b c d : Bool} (H_ac : ∀ (H_b : b), a = c) (H_bd : ∀ (H_c : c), b = d) : a ∧ b ↔ c ∧ d
|
||
:= congr2 not (imp_congrr (λ (H_nnb : ¬ ¬ b), H_ac (not_not_elim H_nnb)) (λ H_c : c, congr2 not (H_bd H_c)))
|
||
theorem and_congrl {a b c d : Bool} (H_bd : ∀ (H_a : a), b = d) (H_ac : ∀ (H_d : d), a = c) : a ∧ b ↔ c ∧ d
|
||
:= congr2 not (imp_congrl (λ H_a : a, congr2 not (H_bd H_a)) (λ (H_nnd : ¬ ¬ d), H_ac (not_not_elim H_nnd)))
|
||
-- (Common case) simplify a to c, and then b to d using the fact that c is true
|
||
theorem and_congr {a b c d : Bool} (H_ac : a = c) (H_bd : ∀ (H_c : c), b = d) : a ∧ b ↔ c ∧ d
|
||
:= and_congrr (λ H, H_ac) H_bd
|
||
|
||
theorem not_and (a b : Bool) : ¬ (a ∧ b) ↔ ¬ a ∨ ¬ b
|
||
:= boolext (assume H, or_elim (em a)
|
||
(assume Ha, or_elim (em b)
|
||
(assume Hb, absurd_elim (¬ a ∨ ¬ b) (and_intro Ha Hb) H)
|
||
(assume Hnb, or_intror (¬ a) Hnb))
|
||
(assume Hna, or_introl Hna (¬ b)))
|
||
(assume (H : ¬ a ∨ ¬ b) (N : a ∧ b),
|
||
or_elim H
|
||
(assume Hna, absurd (and_eliml N) Hna)
|
||
(assume Hnb, absurd (and_elimr N) Hnb))
|
||
|
||
theorem not_and_elim {a b : Bool} (H : ¬ (a ∧ b)) : ¬ a ∨ ¬ b
|
||
:= (not_and a b) ◂ H
|
||
|
||
theorem not_or (a b : Bool) : ¬ (a ∨ b) ↔ ¬ a ∧ ¬ b
|
||
:= boolext (assume H, or_elim (em a)
|
||
(assume Ha, absurd_elim (¬ a ∧ ¬ b) (or_introl Ha b) H)
|
||
(assume Hna, or_elim (em b)
|
||
(assume Hb, absurd_elim (¬ a ∧ ¬ b) (or_intror a Hb) H)
|
||
(assume Hnb, and_intro Hna Hnb)))
|
||
(assume (H : ¬ a ∧ ¬ b) (N : a ∨ b),
|
||
or_elim N
|
||
(assume Ha, absurd Ha (and_eliml H))
|
||
(assume Hb, absurd Hb (and_elimr H)))
|
||
|
||
theorem not_or_elim {a b : Bool} (H : ¬ (a ∨ b)) : ¬ a ∧ ¬ b
|
||
:= (not_or a b) ◂ H
|
||
|
||
theorem not_implies (a b : Bool) : ¬ (a → b) ↔ a ∧ ¬ b
|
||
:= calc (¬ (a → b)) = ¬ (¬ a ∨ b) : { imp_or a b }
|
||
... = ¬ ¬ a ∧ ¬ b : not_or (¬ a) b
|
||
... = a ∧ ¬ b : by simp
|
||
|
||
theorem not_implies_elim {a b : Bool} (H : ¬ (a → b)) : a ∧ ¬ b
|
||
:= (not_implies a b) ◂ H
|
||
|
||
theorem a_eq_not_a (a : Bool) : (a = ¬ a) ↔ false
|
||
:= boolext (λ H, or_elim (em a)
|
||
(λ Ha, absurd Ha (subst Ha H))
|
||
(λ Hna, absurd (subst Hna (symm H)) Hna))
|
||
(λ H, false_elim (a = ¬ a) H)
|
||
|
||
theorem a_iff_not_a (a : Bool) : (a ↔ ¬ a) ↔ false
|
||
:= a_eq_not_a a
|
||
|
||
theorem true_eq_false : (true = false) ↔ false
|
||
:= subst (a_eq_not_a true) not_true
|
||
|
||
theorem true_iff_false : (true ↔ false) ↔ false
|
||
:= true_eq_false
|
||
|
||
theorem false_eq_true : (false = true) ↔ false
|
||
:= subst (a_eq_not_a false) not_false
|
||
|
||
theorem false_iff_true : (false ↔ true) ↔ false
|
||
:= false_eq_true
|
||
|
||
theorem a_iff_true (a : Bool) : (a ↔ true) ↔ a
|
||
:= boolext (λ H, eqt_elim H)
|
||
(λ H, eqt_intro H)
|
||
|
||
theorem a_iff_false (a : Bool) : (a ↔ false) ↔ ¬ a
|
||
:= boolext (λ H, eqf_elim H)
|
||
(λ H, eqf_intro H)
|
||
|
||
add_rewrite a_eq_not_a a_iff_not_a true_eq_false true_iff_false false_eq_true false_iff_true a_iff_true a_iff_false
|
||
|
||
theorem not_iff (a b : Bool) : ¬ (a ↔ b) ↔ (¬ a ↔ b)
|
||
:= or_elim (em b)
|
||
(λ Hb, calc (¬ (a ↔ b)) = (¬ (a ↔ true)) : { eqt_intro Hb }
|
||
... = ¬ a : { a_iff_true a }
|
||
... = ¬ a ↔ true : { symm (a_iff_true (¬ a)) }
|
||
... = ¬ a ↔ b : { symm (eqt_intro Hb) })
|
||
(λ Hnb, calc (¬ (a ↔ b)) = (¬ (a ↔ false)) : { eqf_intro Hnb }
|
||
... = ¬ ¬ a : { a_iff_false a }
|
||
... = ¬ a ↔ false : { symm (a_iff_false (¬ a)) }
|
||
... = ¬ a ↔ b : { symm (eqf_intro Hnb) })
|
||
|
||
theorem not_iff_elim {a b : Bool} (H : ¬ (a ↔ b)) : (¬ a) ↔ b
|
||
:= (not_iff a b) ◂ H
|
||
|
||
theorem forall_or_distributer {A : TypeU} (p : Bool) (φ : A → Bool) : (∀ x, p ∨ φ x) = (p ∨ ∀ x, φ x)
|
||
:= boolext
|
||
(assume H : (∀ x, p ∨ φ x),
|
||
or_elim (em p)
|
||
(λ Hp : p, or_introl Hp (∀ x, φ x))
|
||
(λ Hnp : ¬ p, or_intror p (take x,
|
||
resolve1 (H x) Hnp)))
|
||
(assume H : (p ∨ ∀ x, φ x),
|
||
take x,
|
||
or_elim H
|
||
(λ H1 : p, or_introl H1 (φ x))
|
||
(λ H2 : (∀ x, φ x), or_intror p (H2 x)))
|
||
|
||
theorem forall_or_distributel {A : Type} (p : Bool) (φ : A → Bool) : (∀ x, φ x ∨ p) = ((∀ x, φ x) ∨ p)
|
||
:= boolext
|
||
(assume H : (∀ x, φ x ∨ p),
|
||
or_elim (em p)
|
||
(λ Hp : p, or_intror (∀ x, φ x) Hp)
|
||
(λ Hnp : ¬ p, or_introl (take x, resolve2 (H x) Hnp) p))
|
||
(assume H : (∀ x, φ x) ∨ p,
|
||
take x,
|
||
or_elim H
|
||
(λ H1 : (∀ x, φ x), or_introl (H1 x) p)
|
||
(λ H2 : p, or_intror (φ x) H2))
|
||
|
||
theorem forall_and_distribute {A : TypeU} (φ ψ : A → Bool) : (∀ x, φ x ∧ ψ x) ↔ (∀ x, φ x) ∧ (∀ x, ψ x)
|
||
:= boolext
|
||
(assume H : (∀ x, φ x ∧ ψ x),
|
||
and_intro (take x, and_eliml (H x)) (take x, and_elimr (H x)))
|
||
(assume H : (∀ x, φ x) ∧ (∀ x, ψ x),
|
||
take x, and_intro (and_eliml H x) (and_elimr H x))
|
||
|
||
theorem exists_and_distributer {A : TypeU} (p : Bool) (φ : A → Bool) : (∃ x, p ∧ φ x) ↔ p ∧ ∃ x, φ x
|
||
:= boolext
|
||
(assume H : (∃ x, p ∧ φ x),
|
||
obtain (w : A) (Hw : p ∧ φ w), from H,
|
||
and_intro (and_eliml Hw) (exists_intro w (and_elimr Hw)))
|
||
(assume H : (p ∧ ∃ x, φ x),
|
||
obtain (w : A) (Hw : φ w), from (and_elimr H),
|
||
exists_intro w (and_intro (and_eliml H) Hw))
|
||
|
||
|
||
theorem exists_or_distribute {A : TypeU} (φ ψ : A → Bool) : (∃ x, φ x ∨ ψ x) ↔ (∃ x, φ x) ∨ (∃ x, ψ x)
|
||
:= boolext
|
||
(assume H : (∃ x, φ x ∨ ψ x),
|
||
obtain (w : A) (Hw : φ w ∨ ψ w), from H,
|
||
or_elim Hw
|
||
(λ Hw1 : φ w, or_introl (exists_intro w Hw1) (∃ x, ψ x))
|
||
(λ Hw2 : ψ w, or_intror (∃ x, φ x) (exists_intro w Hw2)))
|
||
(assume H : (∃ x, φ x) ∨ (∃ x, ψ x),
|
||
or_elim H
|
||
(λ H1 : (∃ x, φ x),
|
||
obtain (w : A) (Hw : φ w), from H1,
|
||
exists_intro w (or_introl Hw (ψ w)))
|
||
(λ H2 : (∃ x, ψ x),
|
||
obtain (w : A) (Hw : ψ w), from H2,
|
||
exists_intro w (or_intror (φ w) Hw)))
|
||
|
||
theorem eq_exists_intro {A : (Type U)} {P Q : A → Bool} (H : ∀ x : A, P x ↔ Q x) : (∃ x : A, P x) ↔ (∃ x : A, Q x)
|
||
:= boolext
|
||
(assume Hex, obtain w Pw, from Hex, exists_intro w ((H w) ◂ Pw))
|
||
(assume Hex, obtain w Qw, from Hex, exists_intro w ((symm (H w)) ◂ Qw))
|
||
|
||
theorem not_forall (A : (Type U)) (P : A → Bool) : ¬ (∀ x : A, P x) ↔ (∃ x : A, ¬ P x)
|
||
:= boolext
|
||
(assume H, refute (λ N : ¬ (∃ x, ¬ P x),
|
||
absurd (take x, not_not_elim (not_exists_elim N x)) H))
|
||
(assume (H : ∃ x, ¬ P x) (N : ∀ x, P x),
|
||
obtain w Hw, from H,
|
||
absurd (N w) Hw)
|
||
|
||
theorem not_forall_elim {A : (Type U)} {P : A → Bool} (H : ¬ (∀ x : A, P x)) : ∃ x : A, ¬ P x
|
||
:= (not_forall A P) ◂ H
|
||
|
||
theorem exists_and_distributel {A : TypeU} (p : Bool) (φ : A → Bool) : (∃ x, φ x ∧ p) ↔ (∃ x, φ x) ∧ p
|
||
:= calc (∃ x, φ x ∧ p) = (∃ x, p ∧ φ x) : eq_exists_intro (λ x, and_comm (φ x) p)
|
||
... = (p ∧ (∃ x, φ x)) : exists_and_distributer p φ
|
||
... = ((∃ x, φ x) ∧ p) : and_comm p (∃ x, φ x)
|
||
|
||
theorem exists_imp_distribute {A : TypeU} (φ ψ : A → Bool) : (∃ x, φ x → ψ x) ↔ ((∀ x, φ x) → (∃ x, ψ x))
|
||
:= calc (∃ x, φ x → ψ x) = (∃ x, ¬ φ x ∨ ψ x) : eq_exists_intro (λ x, imp_or (φ x) (ψ x))
|
||
... = (∃ x, ¬ φ x) ∨ (∃ x, ψ x) : exists_or_distribute _ _
|
||
... = ¬ (∀ x, φ x) ∨ (∃ x, ψ x) : { symm (not_forall A φ) }
|
||
... = (∀ x, φ x) → (∃ x, ψ x) : symm (imp_or _ _)
|
||
|
||
theorem forall_uninhabited {A : (Type U)} {B : A → Bool} (H : ¬ inhabited A) : ∀ x, B x
|
||
:= refute (λ N : ¬ (∀ x, B x),
|
||
obtain w Hw, from not_forall_elim N,
|
||
absurd (inhabited_intro w) H)
|
||
|
||
theorem allext {A : (Type U)} {B C : A → Bool} (H : ∀ x : A, B x = C x) : (∀ x : A, B x) = (∀ x : A, C x)
|
||
:= boolext
|
||
(assume Hl, take x, (H x) ◂ (Hl x))
|
||
(assume Hr, take x, (symm (H x)) ◂ (Hr x))
|
||
|
||
-- Up to this point, we proved all theorems using just reflexivity, substitution and case (proof by cases)
|
||
|
||
-- Function extensionality
|
||
axiom funext {A : (Type U)} {B : A → (Type U)} {f g : ∀ x : A, B x} (H : ∀ x : A, f x = g x) : f = g
|
||
|
||
-- Eta is a consequence of function extensionality
|
||
theorem eta {A : TypeU} {B : A → TypeU} (f : ∀ x : A, B x) : (λ x : A, f x) = f
|
||
:= funext (λ x : A, refl (f x))
|
||
|
||
-- Epsilon (Hilbert's operator)
|
||
variable eps {A : TypeU} (H : inhabited A) (P : A → Bool) : A
|
||
alias ε : eps
|
||
axiom eps_ax {A : TypeU} (H : inhabited A) {P : A → Bool} (a : A) : P a → P (ε H P)
|
||
|
||
theorem eps_th {A : TypeU} {P : A → Bool} (a : A) : P a → P (ε (inhabited_intro a) P)
|
||
:= assume H : P a, @eps_ax A (inhabited_intro a) P a H
|
||
|
||
theorem eps_singleton {A : TypeU} (H : inhabited A) (a : A) : ε H (λ x, x = a) = a
|
||
:= let P := λ x, x = a,
|
||
Ha : P a := refl a
|
||
in eps_ax H a Ha
|
||
|
||
-- A function space (∀ x : A, B x) is inhabited if forall a : A, we have inhabited (B a)
|
||
theorem inhabited_fun {A : TypeU} {B : A → TypeU} (Hn : ∀ a, inhabited (B a)) : inhabited (∀ x, B x)
|
||
:= inhabited_intro (λ x, ε (Hn x) (λ y, true))
|
||
|
||
theorem exists_to_eps {A : TypeU} {P : A → Bool} (H : ∃ x, P x) : P (ε (inhabited_ex_intro H) P)
|
||
:= obtain (w : A) (Hw : P w), from H,
|
||
eps_ax (inhabited_ex_intro H) w Hw
|
||
|
||
theorem axiom_of_choice {A : TypeU} {B : A → TypeU} {R : ∀ x : A, B x → Bool} (H : ∀ x, ∃ y, R x y) : ∃ f, ∀ x, R x (f x)
|
||
:= exists_intro
|
||
(λ x, ε (inhabited_ex_intro (H x)) (λ y, R x y)) -- witness for f
|
||
(λ x, exists_to_eps (H x)) -- proof that witness satisfies ∀ x, R x (f x)
|
||
|
||
theorem skolem_th {A : TypeU} {B : A → TypeU} {P : ∀ x : A, B x → Bool} :
|
||
(∀ x, ∃ y, P x y) ↔ ∃ f, (∀ x, P x (f x))
|
||
:= iff_intro
|
||
(λ H : (∀ x, ∃ y, P x y), axiom_of_choice H)
|
||
(λ H : (∃ f, (∀ x, P x (f x))),
|
||
take x, obtain (fw : ∀ x, B x) (Hw : ∀ x, P x (fw x)), from H,
|
||
exists_intro (fw x) (Hw x))
|
||
|
||
-- if-then-else expression, we define it using Hilbert's operator
|
||
definition ite {A : TypeU} (c : Bool) (a b : A) : A
|
||
:= ε (inhabited_intro a) (λ r, (c → r = a) ∧ (¬ c → r = b))
|
||
notation 45 if _ then _ else _ : ite
|
||
|
||
theorem if_true {A : TypeU} (a b : A) : (if true then a else b) = a
|
||
:= calc (if true then a else b) = ε (inhabited_intro a) (λ r, (true → r = a) ∧ (¬ true → r = b)) : refl (if true then a else b)
|
||
... = ε (inhabited_intro a) (λ r, r = a) : by simp
|
||
... = a : eps_singleton (inhabited_intro a) a
|
||
|
||
theorem if_false {A : TypeU} (a b : A) : (if false then a else b) = b
|
||
:= calc (if false then a else b) = ε (inhabited_intro a) (λ r, (false → r = a) ∧ (¬ false → r = b)) : refl (if false then a else b)
|
||
... = ε (inhabited_intro a) (λ r, r = b) : by simp
|
||
... = b : eps_singleton (inhabited_intro a) b
|
||
|
||
theorem if_a_a {A : TypeU} (c : Bool) (a: A) : (if c then a else a) = a
|
||
:= or_elim (em c)
|
||
(λ H : c, calc (if c then a else a) = (if true then a else a) : { eqt_intro H }
|
||
... = a : if_true a a)
|
||
(λ H : ¬ c, calc (if c then a else a) = (if false then a else a) : { eqf_intro H }
|
||
... = a : if_false a a)
|
||
|
||
add_rewrite if_true if_false if_a_a
|
||
|
||
theorem if_congr {A : TypeU} {b c : Bool} {x y u v : A} (H_bc : b = c)
|
||
(H_xu : ∀ (H_c : c), x = u) (H_yv : ∀ (H_nc : ¬ c), y = v) :
|
||
(if b then x else y) = if c then u else v
|
||
:= or_elim (em c)
|
||
(λ H_c : c, calc
|
||
(if b then x else y) = if c then x else y : { H_bc }
|
||
... = if true then x else y : { eqt_intro H_c }
|
||
... = x : if_true _ _
|
||
... = u : H_xu H_c
|
||
... = if true then u else v : symm (if_true _ _)
|
||
... = if c then u else v : { symm (eqt_intro H_c) })
|
||
(λ H_nc : ¬ c, calc
|
||
(if b then x else y) = if c then x else y : { H_bc }
|
||
... = if false then x else y : { eqf_intro H_nc }
|
||
... = y : if_false _ _
|
||
... = v : H_yv H_nc
|
||
... = if false then u else v : symm (if_false _ _)
|
||
... = if c then u else v : { symm (eqf_intro H_nc) })
|
||
|
||
theorem if_imp_then {a b c : Bool} (H : if a then b else c) : a → b
|
||
:= assume Ha : a, eqt_elim (calc b = if true then b else c : symm (if_true b c)
|
||
... = if a then b else c : { symm (eqt_intro Ha) }
|
||
... = true : eqt_intro H)
|
||
|
||
theorem if_imp_else {a b c : Bool} (H : if a then b else c) : ¬ a → c
|
||
:= assume Hna : ¬ a, eqt_elim (calc c = if false then b else c : symm (if_false b c)
|
||
... = if a then b else c : { symm (eqf_intro Hna) }
|
||
... = true : eqt_intro H)
|
||
|
||
theorem app_if_distribute {A B : TypeU} (c : Bool) (f : A → B) (a b : A) : f (if c then a else b) = if c then f a else f b
|
||
:= or_elim (em c)
|
||
(λ Hc : c , calc
|
||
f (if c then a else b) = f (if true then a else b) : { eqt_intro Hc }
|
||
... = f a : { if_true a b }
|
||
... = if true then f a else f b : symm (if_true (f a) (f b))
|
||
... = if c then f a else f b : { symm (eqt_intro Hc) })
|
||
(λ Hnc : ¬ c, calc
|
||
f (if c then a else b) = f (if false then a else b) : { eqf_intro Hnc }
|
||
... = f b : { if_false a b }
|
||
... = if false then f a else f b : symm (if_false (f a) (f b))
|
||
... = if c then f a else f b : { symm (eqf_intro Hnc) })
|
||
|
||
theorem eq_if_distributer {A : TypeU} (c : Bool) (a b v : A) : (v = (if c then a else b)) = if c then v = a else v = b
|
||
:= app_if_distribute c (eq v) a b
|
||
|
||
theorem eq_if_distributel {A : TypeU} (c : Bool) (a b v : A) : ((if c then a else b) = v) = if c then a = v else b = v
|
||
:= app_if_distribute c (λ x, x = v) a b
|
||
|
||
set_opaque exists true
|
||
set_opaque not true
|
||
set_opaque or true
|
||
set_opaque and true
|
||
set_opaque implies true
|
||
set_opaque ite true
|
||
set_opaque eq true
|
||
|
||
definition injective {A B : (Type U)} (f : A → B) := ∀ x1 x2, f x1 = f x2 → x1 = x2
|
||
definition non_surjective {A B : (Type U)} (f : A → B) := ∃ y, ∀ x, ¬ f x = y
|
||
|
||
-- The set of individuals, we need to assert the existence of one infinite set
|
||
variable ind : Type
|
||
-- ind is infinite, i.e., there is a function f s.t. f is injective, and not surjective
|
||
axiom infinity : ∃ f : ind → ind, injective f ∧ non_surjective f
|
||
|
||
-- Pair extensionality
|
||
axiom pairext {A : (Type U)} {B : A → (Type U)} {a b : sig x, B x}
|
||
(H1 : proj1 a = proj1 b) (H2 : proj2 a == proj2 b)
|
||
: a = b
|
||
|
||
-- Proof irrelevance, this is true in the set theoretic model we have for Lean.
|
||
-- In this model, the interpretation of Bool is the set {{*}, {}}.
|
||
-- Thus, if A : Bool is inhabited, then its interpretation must be {*}.
|
||
-- So, the interpretation of H1 and H2 must be *.
|
||
axiom proof_irrel {a b : Bool} (H1 : a) (H2 : b) : H1 == H2
|