-- import Adam.Metadata

-- import Adam.Options.MathlibPart

-- set_option tactic.hygienic false

-- Game "Adam"
-- World "SetTheory"
-- Level 1

-- Title "Mengen"

-- Introduction
-- "
-- In diesem Kapitel schauen wir uns Mengen an.

-- Zuerst ein ganz wichtiger Punkt: Alle Mengen in Lean sind homogen. Das heisst,
-- alle Elemente in einer Menge haben den gleichen Typ.

-- Zum Beispiel `{1, 4, 6}` ist eine Menge von natürlichen Zahlen. Aber man kann keine
-- Menge `{(2 : ℕ), {3, 1}, \"e\", (1 : ℂ)}` definieren, da die Elemente unterschiedliche Typen haben.

-- Für einen Typen `{X : Type*}` definiert damit also `set X` der Type aller Mengen mit Elementen aus
-- `X`.  `set.univ` ist dann ganz `X` also Menge betrachtet, und es ist wichtig den Unterschied
-- zu kennen: `(univ : set X)` und `(X : Typ*)` haben nicht den gleichen Typ und sind damit auch nicht
-- austauschbar!

-- Am anderen Ende sitzt die leere Menge `(∅ : set X)` (`\\empty`). Bei `univ` und `∅` versucht Lean
-- automatisch den Typ zu erraten, in exotischen Beispielen muss man wie oben diesen explizit angeben.

-- Als erste Operationen schauen wir uns `∪` (`\\union` oder `\\cup`), `∩`
-- (`\\inter` oder `\\cap`) und `\\` (`\\\\` oder `\\ `)

-- "

-- open Set

-- Statement
--     "Wenn $B$ wahr ist, dann ist die Implikation $A \\Rightarrow (A ∧ B)$ wahr."
--     {X : Type _} {A B : Set X} : univ \ (A ∩ B) ∪ ∅ = (univ \ A) ∪ (univ \ B) ∪ (A \ B) := by
--   rw [Set.diff_inter]
--   rw [Set.union_empty]
--   rw [Set.union_assoc]
--   rw [←Set.union_diff_distrib]
--   rw [Set.univ_union]

-- NewTactic rw

-- example : 4 ∈ (univ : Set ℕ) := by
--   trivial

-- example (A : Set ℕ) : 4 ∉ (∅ : Set ℕ) := by
--   trivial

-- example (A : Set ℕ) : A ⊆ univ := by
--   intro x h
--   trivial

-- -- -- subset_empty_iff
-- -- example {s : Set α} : s ⊆ ∅ ↔ s = ∅ := by
-- --   constructor
-- --   intro h
-- --   rw [Subset.antisymm_iff]
-- --   constructor
-- --   assumption
-- --   simp only [empty_subset]
-- --   intro a
-- --   rw [Subset.antisymm_iff] at a
-- --   rcases a with ⟨h₁, h₂⟩
-- --   assumption

-- -- -- eq_empty_iff_forall_not_mem
-- -- example {s : Set α} : s = ∅ ↔ ∀ x, x ∉ s := by
-- --   rw [←subset_empty_iff]
-- --   rfl

-- -- -- nonempty_iff_ne_empty
-- -- example (X : Type _) (s : Set X) : Set.Nonempty s ↔ s ≠ ∅ := by
-- --   rw [Set.Nonempty]
-- --   rw [ne_eq, eq_empty_iff_forall_not_mem]
-- --   push_neg
-- --   rfl


-- -- example (A B : ℝ): A = B ↔ A ≤ B ∧ B ≤ A := by library_search

-- namespace Finset

-- theorem powerset_singleton {U : Type _} [DecidableEq U] (x : U) :
--     Finset.powerset {x} = {∅, {x}} := by
--   ext y
--   rw [mem_powerset, subset_singleton_iff, mem_insert, mem_singleton]

-- end Finset

-- /- The powerset of a singleton contains only `∅` and the singleton. -/
-- theorem powerset_singleton {U : Type _} (x : U) :
--     𝒫 ({x} : Set U) = {∅, {x}} := by
--   ext y
--   rw [mem_powerset_iff, subset_singleton_iff_eq]
--   rw [mem_insert_iff, mem_singleton_iff]


-- lemma mem_powerset_insert_iff {U : Type _} (A S : Set U) (x : U) :
--     S ∈ 𝒫 (insert x A) ↔ S \ {x} ∈ 𝒫 A := by
--   rw [mem_powerset_iff, mem_powerset_iff, diff_singleton_subset_iff]

-- -- lemma powerset_insert {U : Type _} (A : Set U) (x : U) :
-- --     𝒫 (insert x A) = 𝒫 A ∪ ({B : Set U | B \ {x} ∈ 𝒫 A}) := by
-- --   sorry

-- theorem subset_insert_iff_of_not_mem {U : Type _ }{s t : Set U} (h : a ∉ s) : s ⊆ insert a t ↔ s ⊆ t := by
--   rw [subset_insert_iff, erase_eq_of_not_mem h]

-- lemma mem_powerset_insert_iff₂ {U : Type _} (A S : Set U) (x : U) :
--     S ∈ 𝒫 (insert x A) ↔ S ∈ 𝒫 A ∨ ∃ B ∈ 𝒫 A , insert x B = S := by
--   simp_rw [mem_powerset_iff]
--   constructor
--   · intro h
--     by_cases hs : x ∈ S
--     · sorry
--     · left
--       rw [Finset.subset_insert_iff_of_not_mem]
--   · intro h
--     rcases h with h | ⟨B, h₁, h₂⟩
--     · exact le_trans h (subset_insert x A)
--     · rw [←h₂]
--       exact insert_subset_insert h₁

-- lemma powerset_insert {U : Type _} (A : Set U) (x : U) :
--     𝒫 (insert x A) = A.powerset ∪ A.powerset.image (insert x) := by
--   rw [Subset.antisymm_iff]
--   constructor <;>
--   intro B hB <;>
--   simp_rw [mem_powerset_insert_iff₂, mem_union, mem_image] at hB ⊢ <;>
--   assumption


-- example : powerset {0, 1} = {∅, {0}, {1}, {0, 1}} := by
--   simp_rw [powerset_insert, powerset_singleton, Finset.powerset_insert, Finset.powerset_singleton]
--   simp only [image_insert_eq, union_insert, image_singleton, union_singleton]
--   simp only [insert_comm, ←insert_emptyc_eq]

-- example [DecidableEq ℕ] : Finset.powerset {0, 1} = {∅, {0}, {1}, {0, 1}} := by
--   repeat' rw [@ Finset.powerset_insert ℕ]
--   repeat' rw [@Finset.powerset_singleton ℕ]
--   --simp only [image_insert_eq, union_insert, image_singleton, union_singleton]


-- example : powerset {0, 1, 2, 4} =
--     {∅, {0}, {1}, {0, 1}, {2}, {1, 2}, {0, 2}, {0, 1, 2},
--     {4}, {0, 4}, {1, 4}, {0, 1, 4}, {2, 4}, {1, 2, 4}, {0, 2, 4}, {0, 1, 2, 4}} := by
--   simp_rw [powerset_insert, powerset_singleton]
--   simp_rw [image_insert_eq, image_singleton]



-- example : Finset.powerset ({0, 1} : Finset ℕ) = {{0, 1}, ∅, {1}, {0}} := by
--   have h : Finset.powerset ({0, 1} : Finset ℕ) = {∅, {0}, {1}, {0, 1}}
--   rfl
--   rw [h]
--   simp only []


-- example : ({∅, {0}, {1}, {0, 1}} : Finset (Finset ℕ)) = {∅, {1}, {0}, {0, 1}} := by
--   simp only []



-- lemma subset_of_subset_diff {U : Type _} (A B C : Set U) (h : A ⊆ B \ C) : A ⊆ B :=
--   fun _ hx => mem_of_mem_diff (h hx)

-- lemma subset_of_subset_diff' {U : Type _} (A B C : Set U) (h : A ⊆ B) : A \ C ⊆ B :=
--   sorry

-- lemma mem_powerset' {U : Type _} {A B C : Set U}
--     (h' : B ∈ 𝒫 C) (h : A ⊆ B) :
--     A ∈ 𝒫 C := by
--   rw [mem_powerset_iff] at h' ⊢
--   exact le_trans h h'

-- example (A B : Set ℕ) : A = B ↔ ∀ x, x ∈ A ↔ x ∈ B := by
--   exact ext_iff



-- lemma NO.powerset_insert {U : Type _} (A : Set U) (x : U) :
--     𝒫 (insert x A) = 𝒫 A ∪ ({insert x B | B ∈ 𝒫 A}) := by
--   ext y
--   rw [mem_powerset_insert_iff]
--   constructor
--   · intro h
--     rw [mem_union, mem_setOf]
--     by_cases hx : x ∈ y
--     · right
--       use y \ {x}
--       constructor
--       · assumption
--       · rw [insert_diff_singleton, insert_eq_of_mem hx]
--     · left
--       rw [diff_singleton_eq_self hx] at h
--       assumption
--   · intro h
--     rw [mem_union, mem_setOf] at h
--     rcases h with h | h
--     · apply mem_powerset' h
--       simp
--     · rcases h with ⟨B, hB⟩
--       rw [←hB.2]
--       apply mem_powerset' hB.1
--       simp

-- -- lemma xxx {U : Type _} (x y : U):
-- --     ({insert x B | B ∈ {∅, }}) = {({x} : Set U), {x, y}} := by
-- --   sorry

-- -- lemma hh {U : Type _} (A : Set U) (x : U) :
-- --     A \ {x} ∈

-- lemma diff_singleton_eq_iff {U : Type _} {A B : Set U} {x : U} :
--     A \ {x} = B ↔ A = B ∨ A = insert x B := by sorry

-- lemma x1 {U : Type _} (x : U) : insert x (∅ : Set U) = {x} := by sorry

-- --set_option maxHeartbeats 20000


-- example {U : Type _} {x y : U} : ({x, y} : Set U) = {y, x} := by
--   exact pair_comm x y

-- example {U : Type _} {A : Set U} {x y : U} : insert x (insert y A) = insert y (insert x A) := by
--   exact insert_comm x y A

-- open Classical

-- #check decide

-- example : ({{0, 2}, {3, 5, 6}} : Set (Set ℕ)) = {{2, 0}, {5, 3, 6}} := by
--   rw [Subset.antisymm_iff]
--   constructor <;>
--   intro A hA <;>
--   simp_rw [mem_insert_iff, mem_singleton_iff] at *
--   · rw [pair_comm 2 0, insert_comm 5 3]
--     tauto
--   · rw [pair_comm 0 2, insert_comm 3 5]
--     tauto

-- example (A : Set ℕ) : ({{0, 2}, A, {3, 5, 6}} : Set (Set ℕ)) = {{2, 0}, {5, 3, 6}, A} := by
--   simp only [insert_comm, ←insert_emptyc_eq]


-- example : ({{0, 2}, {3, 5, 6}} : Finset (Finset ℕ)) = {{2, 0}, {5, 3, 6}} := by
--   simp only


--   -- -- This works but does not scale well
--   -- ext x
--   -- simp_rw [powerset_insert, powerset_singleton]
--   -- simp_rw [mem_union, mem_setOf, mem_insert_iff, mem_singleton_iff]
--   -- simp_rw [diff_singleton_eq_iff, x1]
--   -- tauto




--   -- rw [Subset.antisymm_iff]
--   -- constructor
--   -- intro A hA
--   -- rw [mem_powerset_iff] at hA
--   -- simp_rw [mem_insert_iff, mem_singleton_iff] at *


-- example : ({2, 3, 5} : Set ℕ) ⊆ {4, 2, 5, 7, 3} := by
--   intro a ha
--   simp_rw [Set.mem_insert_iff, Set.mem_singleton_iff] at *
--   tauto

-- example : ({0} : Set ℕ) ∪ {1, 2} = {0, 2, 1} := by
--   rw [Subset.antisymm_iff]
--   intro A hA
--   --rw [Set.mem_insert_iff]



-- -- Trick:
-- -- attribute [default_instance] Set.instSingletonSet