-- Mathlib/Combinatorics/Hindman.lean
theorem exists_FP_of_large {M} [Semigroup M] (U : Ultrafilter M) (U_idem : U * U = U) (s₀ : Set M)
    (sU : s₀ ∈ U) : ∃ a, FP a ⊆ s₀

Proof state:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
⊢ ∃ a, FP a ⊆ s₀

Tactic:
use Stream'.corec elem succ (Subtype.mk s₀ sU)

Result:
case h
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
⊢ FP (Stream'.corec elem succ ⟨s₀, sU⟩) ⊆ s₀

Proof state:
case h.head
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m : M
b : Stream' M
⊢ ∀ (p : { s // s ∈ U }), b = Stream'.corec elem succ p → b.head ∈ ↑p

Tactic:
rintro p rfl

Result:
case h.head
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m : M
p : { s // s ∈ U }
⊢ (Stream'.corec elem succ p).head ∈ ↑p

Proof state:
case h.head
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m : M
p : { s // s ∈ U }
⊢ elem p ∈ ↑p

Tactic:
exact Set.inter_subset_left (Set.Nonempty.some_mem _)

Result:
no goals

Proof state:
case h.cons.refine_2
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m n : M
p : { s // s ∈ U }
h : FP (Stream'.corec elem succ p).tail n
ih : ∀ (p_1 : { s // s ∈ U }), (Stream'.corec elem succ p).tail = Stream'.corec elem succ p_1 → n ∈ ↑p_1
this : n ∈ {m | elem p * m ∈ ↑p}
⊢ (Stream'.corec elem succ p).head * n ∈ ↑p

Tactic:
simpa only using this

Result:
no goals

Proof state:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
⊢ ∃ a, FP a ⊆ s₀

Tactic:
have exists_elem : ∀ {s : Set M} (_hs : s ∈ U), (s ∩ { m | ∀ᶠ m' in U, m * m' ∈ s }).Nonempty :=
  fun {s} hs => Ultrafilter.nonempty_of_mem (inter_mem hs <| by rwa [← U_idem] at hs)

Result:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
⊢ ∃ a, FP a ⊆ s₀

Proof state:
case h
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
⊢ ∀ (a : Stream' M), ∀ m ∈ FP a, ∀ (p : { s // s ∈ U }), a = Stream'.corec elem succ p → m ∈ ↑p

Tactic:
intro a m h

Result:
case h
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m : M
h : m ∈ FP a
⊢ ∀ (p : { s // s ∈ U }), a = Stream'.corec elem succ p → m ∈ ↑p

Proof state:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
this : ∀ (a : Stream' M), ∀ m ∈ FP a, ∀ (p : { s // s ∈ U }), a = Stream'.corec elem succ p → m ∈ ↑p
⊢ FP (Stream'.corec elem succ ⟨s₀, sU⟩) ⊆ s₀

Tactic:
intro m hm

Result:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
this : ∀ (a : Stream' M), ∀ m ∈ FP a, ∀ (p : { s // s ∈ U }), a = Stream'.corec elem succ p → m ∈ ↑p
m : M
hm : m ∈ FP (Stream'.corec elem succ ⟨s₀, sU⟩)
⊢ m ∈ s₀

Proof state:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
this : ∀ (a : Stream' M), ∀ m ∈ FP a, ∀ (p : { s // s ∈ U }), a = Stream'.corec elem succ p → m ∈ ↑p
m : M
hm : m ∈ FP (Stream'.corec elem succ ⟨s₀, sU⟩)
⊢ m ∈ s₀

Tactic:
exact this _ m hm ⟨s₀, sU⟩ rfl

Result:
no goals

Proof state:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
⊢ ∃ a, FP a ⊆ s₀

Tactic:
let succ : {s // s ∈ U} → {s // s ∈ U} := fun (p : {s // s ∈ U}) =>
      ⟨p.val ∩ {m : M | elem p * m ∈ p.val},
       inter_mem p.property
         (show (exists_elem p.property).some ∈ {m : M | ∀ᶠ (m' : M) in ↑U, m * m' ∈ p.val} from
            p.val.inter_subset_right (exists_elem p.property).some_mem)⟩

Result:
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
s₀ : Set M
sU : s₀ ∈ U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
⊢ ∃ a, FP a ⊆ s₀

Proof state:
case h.cons
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m : M
b : Stream' M
n : M
h : FP b.tail n
ih : ∀ (p : { s // s ∈ U }), b.tail = Stream'.corec elem succ p → n ∈ ↑p
⊢ ∀ (p : { s // s ∈ U }), b = Stream'.corec elem succ p → b.head * n ∈ ↑p

Tactic:
rintro p rfl

Result:
case h.cons
M : Type u_1
inst✝ : Semigroup M
U : Ultrafilter M
U_idem : U * U = U
exists_elem : ∀ {s : Set M}, s ∈ U → (s ∩ {m | ∀ᶠ (m' : M) in ↑U, m * m' ∈ s}).Nonempty
elem : { s // s ∈ U } → M := fun p => ⋯.some
succ : { s // s ∈ U } → { s // s ∈ U } := fun p => ⟨↑p ∩ {m | elem p * m ∈ ↑p}, ⋯⟩
a : Stream' M
m n : M
p : { s // s ∈ U }
h : FP (Stream'.corec elem succ p).tail n
ih : ∀ (p_1 : { s // s ∈ U }), (Stream'.corec elem succ p).tail = Stream'.corec elem succ p_1 → n ∈ ↑p_1
⊢ (Stream'.corec elem succ p).head * n ∈ ↑p