-- Mathlib/ModelTheory/Fraisse.lean
theorem exists_cg_is_age_of (hn : K.Nonempty)
    (hc : (Quotient.mk' '' K).Countable)
    (fg : ∀ M : Bundled.{w} L.Structure, M ∈ K → Structure.FG L M) (hp : Hereditary K)
    (jep : JointEmbedding K) : ∃ M : Bundled.{w} L.Structure, Structure.CG L M ∧ L.age M = K

Proof state:
case intro.intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
n : ℕ
P : Bundled L.Structure
hP1 : P ∈ K
hP2 : (Quotient.mk' P).out ≈ (F n).out
⊢ (F n).out ∈ K

Tactic:
replace hP2 := Setoid.trans (Setoid.symm (Quotient.mk_out P)) hP2

Result:
case intro.intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
n : ℕ
P : Bundled L.Structure
hP1 : P ∈ K
hP2 : P ≈ (F n).out
⊢ (F n).out ∈ K

Proof state:
case intro.refine_2
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
f : (i j : ℕ) → i ≤ j → ↑↑(G i) ↪[L] ↑↑(G j) := DirectedSystem.natLERec (id fun n => Nonempty.some ⋯)
this✝ : DirectedSystem (fun n => ↑↑(G n)) fun i j h => ⇑(f i j h)
N : Bundled L.Structure
KN : N ∈ K
this : (Quotient.mk' N).out ≈ N
⊢ N ∈ ⋃ i, L.age ↑↑(G i)

Tactic:
obtain ⟨n, ⟨e⟩⟩ := (hF N).1 ⟨N, KN, this⟩

Result:
case intro.refine_2.intro.intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
f : (i j : ℕ) → i ≤ j → ↑↑(G i) ↪[L] ↑↑(G j) := DirectedSystem.natLERec (id fun n => Nonempty.some ⋯)
this✝ : DirectedSystem (fun n => ↑↑(G n)) fun i j h => ⇑(f i j h)
N : Bundled L.Structure
KN : N ∈ K
this : (Quotient.mk' N).out ≈ N
n : ℕ
e : ↑(F n).out ≃[L] ↑N
⊢ N ∈ ⋃ i, L.age ↑↑(G i)

Proof state:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
f : (i j : ℕ) → i ≤ j → ↑↑(G i) ↪[L] ↑↑(G j) := DirectedSystem.natLERec (id fun n => Nonempty.some ⋯)
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K

Tactic:
have : DirectedSystem (fun n ↦ (G n).val) fun i j h ↦ ↑(f i j h) := by
  dsimp [f, G]; infer_instance

Result:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
f : (i j : ℕ) → i ≤ j → ↑↑(G i) ↪[L] ↑↑(G j) := DirectedSystem.natLERec (id fun n => Nonempty.some ⋯)
this : DirectedSystem (fun n => ↑↑(G n)) fun i j h => ⇑(f i j h)
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K

Proof state:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K

Tactic:
let G : ℕ → K := @Nat.rec (fun _ => K) ⟨(F 0).out, hF' 0⟩ fun n N => ⟨P N n, hPK N n⟩

Result:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K

Proof state:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K

Tactic:
let f : ∀ (i j : ℕ), i ≤ j → (G i).val ↪[L] (G j).val := by
  refine DirectedSystem.natLERec (G' := fun i => (G i).val) (L := L) ?_
  dsimp only [G]
  exact fun n => (hP _ n).some

Result:
case intro
L : Language
K : Set (Bundled L.Structure)
hn : K.Nonempty
hc : (Quotient.mk' '' K).Countable
fg : ∀ M ∈ K, Structure.FG L ↑M
hp : Hereditary K
jep : JointEmbedding K
F : ℕ → Quotient equivSetoid
hF : ∀ (a : Bundled L.Structure), (∃ x ∈ K, (Quotient.mk' x).out ≈ a) ↔ ∃ y, (F y).out ≈ a
hF' : ∀ (n : ℕ), (F n).out ∈ K
P : ↑K → ℕ → Bundled L.Structure
hPK : ∀ (N : ↑K) (n : ℕ), P N n ∈ K
hP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (↑N) (P N n)
hFP : ∀ (N : ↑K) (n : ℕ), (fun M N => Nonempty (↑M ↪[L] ↑N)) (F (n + 1)).out (P N n)
G : ℕ → ↑K := Nat.rec ⟨(F 0).out, ⋯⟩ fun n N => ⟨P N n, ⋯⟩
f : (i j : ℕ) → i ≤ j → ↑↑(G i) ↪[L] ↑↑(G j) := DirectedSystem.natLERec (id fun n => Nonempty.some ⋯)
⊢ ∃ M, Structure.CG L ↑M ∧ L.age ↑M = K