65 lines
3.2 KiB
Text
65 lines
3.2 KiB
Text
-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
|
|
-- Released under Apache 2.0 license as described in the file LICENSE.
|
|
-- Author: Leonardo de Moura
|
|
import data.sigma.decl logic.wf logic.cast
|
|
open well_founded
|
|
|
|
namespace sigma
|
|
section
|
|
variables {A : Type} {B : A → Type}
|
|
variable (Ra : A → A → Prop)
|
|
variable (Rb : ∀ a, B a → B a → Prop)
|
|
|
|
-- Lexicographical order based on Ra and Rb
|
|
inductive lex : sigma B → sigma B → Prop :=
|
|
left : ∀{a₁ b₁} a₂ b₂, Ra a₁ a₂ → lex (dpair a₁ b₁) (dpair a₂ b₂),
|
|
right : ∀a {b₁ b₂}, Rb a b₁ b₂ → lex (dpair a b₁) (dpair a b₂)
|
|
end
|
|
|
|
context
|
|
parameters {A : Type} {B : A → Type}
|
|
parameters {Ra : A → A → Prop} {Rb : Π a : A, B a → B a → Prop}
|
|
infix `≺`:50 := lex Ra Rb
|
|
|
|
set_option pp.beta true
|
|
|
|
definition lex.accessible {a} (aca : acc Ra a) (acb : ∀a, well_founded (Rb a)) : ∀ (b : B a), acc (lex Ra Rb) (dpair a b) :=
|
|
acc.rec_on aca
|
|
(λxa aca (iHa : ∀y, Ra y xa → ∀b : B y, acc (lex Ra Rb) (dpair y b)),
|
|
λb : B xa, acc.rec_on (acb xa b)
|
|
(λxb acb
|
|
(iHb : ∀y, Rb xa y xb → acc (lex Ra Rb) (dpair xa y)),
|
|
acc.intro (dpair xa xb) (λp (lt : p ≺ (dpair xa xb)),
|
|
have aux : xa = xa → xb == xb → acc (lex Ra Rb) p, from
|
|
@lex.rec_on A B Ra Rb (λp₁ p₂, dpr1 p₂ = xa → dpr2 p₂ == xb → acc (lex Ra Rb) p₁)
|
|
p (dpair xa xb) lt
|
|
(λa₁ b₁ a₂ b₂ (H : Ra a₁ a₂) (eq₂ : a₂ = xa) (eq₃ : b₂ == xb),
|
|
show acc (lex Ra Rb) (dpair a₁ b₁), from
|
|
have Ra₁ : Ra a₁ xa, from eq.rec_on eq₂ H,
|
|
iHa a₁ Ra₁ b₁)
|
|
(λa b₁ b₂ (H : Rb a b₁ b₂) (eq₂ : a = xa) (eq₃ : b₂ == xb),
|
|
-- TODO(Leo): cleanup this proof
|
|
show acc (lex Ra Rb) (dpair a b₁), from
|
|
let b₁' : B xa := eq.rec_on eq₂ b₁ in
|
|
have aux₁ : b₁ == b₁', from
|
|
heq.symm (eq_rec_heq eq₂ b₁),
|
|
have aux₂ : Rb a b₁ b₂ = Rb xa b₁' xb, from
|
|
heq.to_eq (hcongr_arg3 Rb eq₂ aux₁ eq₃),
|
|
have aux₃ : Rb xa b₁' xb, from
|
|
eq.rec_on aux₂ H,
|
|
have aux₄ : acc (lex Ra Rb) (dpair xa b₁'), from
|
|
iHb b₁' aux₃,
|
|
have aux₅ : ∀ (b₁ b₂ : B a) (H₁ : a = a) (H₂ : b₁ == b₂), acc (lex Ra Rb) (dpair a b₁) → acc (lex Ra Rb) (dpair a b₂), from
|
|
λ b₁ b₂ H₁ H₂ Ha, eq.rec_on (heq.to_eq H₂) Ha,
|
|
have aux₆ : ∀ (b₁ : B xa) (b₂ : B a) (H₁ : a = xa) (H₂ : b₁ == b₂), acc (lex Ra Rb) (dpair xa b₁) → acc (lex Ra Rb) (dpair a b₂), from
|
|
eq.rec_on eq₂ aux₅,
|
|
aux₆ b₁' b₁ eq₂ (heq.symm aux₁) aux₄),
|
|
aux rfl !heq.refl)))
|
|
|
|
-- The lexicographical order of well founded relations is well-founded
|
|
definition lex.wf (Ha : well_founded Ra) (Hb : ∀ x, well_founded (Rb x)) : well_founded (lex Ra Rb) :=
|
|
well_founded.intro (λp, destruct p (λa b, lex.accessible (Ha a) Hb b))
|
|
|
|
end
|
|
|
|
end sigma
|