284 lines
9.6 KiB
Text
284 lines
9.6 KiB
Text
/-
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Copyright (c) 2014 Parikshit Khanna. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Module: data.list.basic
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Authors: Parikshit Khanna, Jeremy Avigad, Leonardo de Moura
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Basic properties of lists.
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-/
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import logic tools.helper_tactics data.nat.basic
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open eq.ops helper_tactics nat
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inductive list (T : Type) : Type :=
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| nil {} : list T
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| cons : T → list T → list T
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namespace list
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notation h :: t := cons h t
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notation `[` l:(foldr `,` (h t, cons h t) nil) `]` := l
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variable {T : Type}
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/- append -/
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definition append : list T → list T → list T
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| append nil l := l
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| append (h :: s) t := h :: (append s t)
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notation l₁ ++ l₂ := append l₁ l₂
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theorem append_nil_left (t : list T) : nil ++ t = t
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theorem append_cons (x : T) (s t : list T) : (x::s) ++ t = x::(s ++ t)
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theorem append_nil_right : ∀ (t : list T), t ++ nil = t
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| append_nil_right nil := rfl
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| append_nil_right (a :: l) := calc
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(a :: l) ++ nil = a :: (l ++ nil) : rfl
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... = a :: l : append_nil_right l
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theorem append.assoc : ∀ (s t u : list T), s ++ t ++ u = s ++ (t ++ u)
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| append.assoc nil t u := rfl
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| append.assoc (a :: l) t u :=
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begin
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change a :: (l ++ t ++ u) = (a :: l) ++ (t ++ u),
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rewrite append.assoc
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end
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/- length -/
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definition length : list T → nat
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| length nil := 0
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| length (a :: l) := length l + 1
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theorem length_nil : length (@nil T) = 0
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theorem length_cons (x : T) (t : list T) : length (x::t) = length t + 1
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theorem length_append : ∀ (s t : list T), length (s ++ t) = length s + length t
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| length_append nil t := calc
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length (nil ++ t) = length t : rfl
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... = length nil + length t : zero_add
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| length_append (a :: s) t := calc
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length (a :: s ++ t) = length (s ++ t) + 1 : rfl
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... = length s + length t + 1 : length_append
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... = (length s + 1) + length t : add.succ_left
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... = length (a :: s) + length t : rfl
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-- add_rewrite length_nil length_cons
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/- concat -/
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definition concat : Π (x : T), list T → list T
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| concat a nil := [a]
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| concat a (b :: l) := b :: concat a l
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theorem concat_nil (x : T) : concat x nil = [x]
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theorem concat_cons (x y : T) (l : list T) : concat x (y::l) = y::(concat x l)
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theorem concat_eq_append (a : T) : ∀ (l : list T), concat a l = l ++ [a]
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| concat_eq_append nil := rfl
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| concat_eq_append (b :: l) :=
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begin
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change b :: (concat a l) = (b :: l) ++ (a :: nil),
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rewrite concat_eq_append
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end
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-- add_rewrite append_nil append_cons
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/- reverse -/
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definition reverse : list T → list T
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| reverse nil := nil
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| reverse (a :: l) := concat a (reverse l)
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theorem reverse_nil : reverse (@nil T) = nil
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theorem reverse_cons (x : T) (l : list T) : reverse (x::l) = concat x (reverse l)
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theorem reverse_singleton (x : T) : reverse [x] = [x]
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theorem reverse_append : ∀ (s t : list T), reverse (s ++ t) = (reverse t) ++ (reverse s)
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| reverse_append nil t2 := calc
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reverse (nil ++ t2) = reverse t2 : rfl
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... = (reverse t2) ++ nil : append_nil_right
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... = (reverse t2) ++ (reverse nil) : {reverse_nil⁻¹}
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| reverse_append (a2 :: s2) t2 := calc
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reverse ((a2 :: s2) ++ t2) = concat a2 (reverse (s2 ++ t2)) : rfl
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... = concat a2 (reverse t2 ++ reverse s2) : reverse_append
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... = (reverse t2 ++ reverse s2) ++ [a2] : concat_eq_append
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... = reverse t2 ++ (reverse s2 ++ [a2]) : append.assoc
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... = reverse t2 ++ concat a2 (reverse s2) : concat_eq_append
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... = reverse t2 ++ reverse (a2 :: s2) : rfl
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theorem reverse_reverse : ∀ (l : list T), reverse (reverse l) = l
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| reverse_reverse nil := rfl
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| reverse_reverse (a :: l) := calc
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reverse (reverse (a :: l)) = reverse (concat a (reverse l)) : rfl
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... = reverse (reverse l ++ [a]) : concat_eq_append
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... = reverse [a] ++ reverse (reverse l) : reverse_append
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... = reverse [a] ++ l : reverse_reverse
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... = a :: l : rfl
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theorem concat_eq_reverse_cons (x : T) (l : list T) : concat x l = reverse (x :: reverse l) :=
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calc
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concat x l = concat x (reverse (reverse l)) : reverse_reverse
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... = reverse (x :: reverse l) : rfl
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/- head and tail -/
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definition head [h : inhabited T] : list T → T
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| head nil := arbitrary T
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| head (a :: l) := a
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theorem head_cons [h : inhabited T] (a : T) (l : list T) : head (a::l) = a
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theorem head_concat [h : inhabited T] (t : list T) : ∀ {s : list T}, s ≠ nil → head (s ++ t) = head s
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| @head_concat nil H := absurd rfl H
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| @head_concat (a :: s) H :=
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begin
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change head (a :: (s ++ t)) = head (a :: s),
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rewrite head_cons
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end
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definition tail : list T → list T
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| tail nil := nil
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| tail (a :: l) := l
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theorem tail_nil : tail (@nil T) = nil
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theorem tail_cons (a : T) (l : list T) : tail (a::l) = l
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theorem cons_head_tail [h : inhabited T] {l : list T} : l ≠ nil → (head l)::(tail l) = l :=
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list.cases_on l
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(assume H : nil ≠ nil, absurd rfl H)
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(take x l, assume H : x::l ≠ nil, rfl)
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/- list membership -/
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definition mem : T → list T → Prop
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| mem a nil := false
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| mem a (b :: l) := a = b ∨ mem a l
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notation e ∈ s := mem e s
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theorem mem_nil (x : T) : x ∈ nil ↔ false :=
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iff.rfl
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theorem mem_cons (x y : T) (l : list T) : x ∈ y::l ↔ (x = y ∨ x ∈ l) :=
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iff.rfl
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theorem mem_concat_imp_or {x : T} {s t : list T} : x ∈ s ++ t → x ∈ s ∨ x ∈ t :=
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list.induction_on s or.inr
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(take y s,
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assume IH : x ∈ s ++ t → x ∈ s ∨ x ∈ t,
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assume H1 : x ∈ y::s ++ t,
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have H2 : x = y ∨ x ∈ s ++ t, from H1,
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have H3 : x = y ∨ x ∈ s ∨ x ∈ t, from or_of_or_of_imp_right H2 IH,
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iff.elim_right or.assoc H3)
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theorem mem_or_imp_concat {x : T} {s t : list T} : x ∈ s ∨ x ∈ t → x ∈ s ++ t :=
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list.induction_on s
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(take H, or.elim H false.elim (assume H, H))
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(take y s,
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assume IH : x ∈ s ∨ x ∈ t → x ∈ s ++ t,
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assume H : x ∈ y::s ∨ x ∈ t,
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or.elim H
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(assume H1,
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or.elim H1
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(take H2 : x = y, or.inl H2)
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(take H2 : x ∈ s, or.inr (IH (or.inl H2))))
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(assume H1 : x ∈ t, or.inr (IH (or.inr H1))))
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theorem mem_concat (x : T) (s t : list T) : x ∈ s ++ t ↔ x ∈ s ∨ x ∈ t :=
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iff.intro mem_concat_imp_or mem_or_imp_concat
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local attribute mem [reducible]
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local attribute append [reducible]
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theorem mem_split {x : T} {l : list T} : x ∈ l → ∃s t : list T, l = s ++ (x::t) :=
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list.induction_on l
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(take H : x ∈ nil, false.elim (iff.elim_left !mem_nil H))
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(take y l,
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assume IH : x ∈ l → ∃s t : list T, l = s ++ (x::t),
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assume H : x ∈ y::l,
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or.elim H
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(assume H1 : x = y,
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exists.intro nil (!exists.intro (H1 ▸ rfl)))
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(assume H1 : x ∈ l,
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obtain s (H2 : ∃t : list T, l = s ++ (x::t)), from IH H1,
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obtain t (H3 : l = s ++ (x::t)), from H2,
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have H4 : y :: l = (y::s) ++ (x::t),
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from H3 ▸ rfl,
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!exists.intro (!exists.intro H4)))
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definition decidable_mem [instance] [H : decidable_eq T] (x : T) (l : list T) : decidable (x ∈ l) :=
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list.rec_on l
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(decidable.inr (not_of_iff_false !mem_nil))
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(take (h : T) (l : list T) (iH : decidable (x ∈ l)),
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show decidable (x ∈ h::l), from
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decidable.rec_on iH
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(assume Hp : x ∈ l,
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decidable.rec_on (H x h)
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(assume Heq : x = h,
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decidable.inl (or.inl Heq))
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(assume Hne : x ≠ h,
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decidable.inl (or.inr Hp)))
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(assume Hn : ¬x ∈ l,
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decidable.rec_on (H x h)
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(assume Heq : x = h,
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decidable.inl (or.inl Heq))
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(assume Hne : x ≠ h,
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have H1 : ¬(x = h ∨ x ∈ l), from
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assume H2 : x = h ∨ x ∈ l, or.elim H2
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(assume Heq, absurd Heq Hne)
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(assume Hp, absurd Hp Hn),
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have H2 : ¬x ∈ h::l, from
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iff.elim_right (not_iff_not_of_iff !mem_cons) H1,
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decidable.inr H2)))
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/- find -/
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section
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variable [H : decidable_eq T]
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include H
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definition find : T → list T → nat
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| find a nil := 0
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| find a (b :: l) := if a = b then 0 else succ (find a l)
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theorem find_nil (x : T) : find x nil = 0
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theorem find_cons (x y : T) (l : list T) : find x (y::l) = if x = y then 0 else succ (find x l)
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theorem find.not_mem {l : list T} {x : T} : ¬x ∈ l → find x l = length l :=
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list.rec_on l
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(assume P₁ : ¬x ∈ nil, _)
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(take y l,
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assume iH : ¬x ∈ l → find x l = length l,
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assume P₁ : ¬x ∈ y::l,
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have P₂ : ¬(x = y ∨ x ∈ l), from iff.elim_right (not_iff_not_of_iff !mem_cons) P₁,
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have P₃ : ¬x = y ∧ ¬x ∈ l, from (iff.elim_left not_or_iff_not_and_not P₂),
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calc
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find x (y::l) = if x = y then 0 else succ (find x l) : !find_cons
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... = succ (find x l) : if_neg (and.elim_left P₃)
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... = succ (length l) : {iH (and.elim_right P₃)}
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... = length (y::l) : !length_cons⁻¹)
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end
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/- nth element -/
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definition nth [h : inhabited T] : list T → nat → T
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| nth nil n := arbitrary T
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| nth (a :: l) 0 := a
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| nth (a :: l) (n+1) := nth l n
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theorem nth_zero [h : inhabited T] (a : T) (l : list T) : nth (a :: l) 0 = a
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theorem nth_succ [h : inhabited T] (a : T) (l : list T) (n : nat) : nth (a::l) (n+1) = nth l n
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end list
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