-- Mathlib/Data/Nat/Find.lean
lemma findGreatest_eq_iff :
    Nat.findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n⦄, m < n → n ≤ k → ¬P n

Proof state:
case pos.mp
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
hk : P (k + 1)
⊢ k + 1 ≤ k + 1 ∧ (k + 1 ≠ 0 → P (k + 1)) ∧ ∀ ⦃n : ℕ⦄, k + 1 < n → n ≤ k + 1 → ¬P n

Tactic:
exact ⟨le_refl _, fun _ ↦ hk, fun n hlt hle ↦ by omega⟩

Result:
no goals

Proof state:
case pos
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ k + 1 = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Tactic:
constructor

Result:
case pos.mp
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ k + 1 = m → m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

case pos.mpr
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ (m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n) → k + 1 = m

Proof state:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
⊢ (¬0 = 0 → P 0) ∧ ∀ ⦃n : ℕ⦄, 0 < n → n = 0 → ¬P n

Tactic:
exact ⟨fun h ↦ (h rfl).elim, fun n hlt heq ↦ by omega⟩

Result:
no goals

Proof state:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
m : ℕ
⊢ m = 0 → (¬m = 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n = 0 → ¬P n

Tactic:
rintro rfl

Result:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
⊢ (¬0 = 0 → P 0) ∧ ∀ ⦃n : ℕ⦄, 0 < n → n = 0 → ¬P n

Proof state:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
m : ℕ
⊢ findGreatest P 0 = m ↔ m ≤ 0 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ 0 → ¬P n

Tactic:
rw [eq_comm, Iff.comm]

Result:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
m : ℕ
⊢ (m ≤ 0 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ 0 → ¬P n) ↔ m = findGreatest P 0

Proof state:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
m : ℕ
⊢ (m ≤ 0 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ 0 → ¬P n) ↔ m = findGreatest P 0

Tactic:
simp only [zero_eq, Nat.le_zero, ne_eq, findGreatest_zero, and_iff_left_iff_imp]

Result:
case zero
P : ℕ → Prop
inst✝ : DecidablePred P
m : ℕ
⊢ m = 0 → (¬m = 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n = 0 → ¬P n

Proof state:
P : ℕ → Prop
inst✝ : DecidablePred P
n : ℕ
hlt : 0 < n
heq : n = 0
⊢ ¬P n

Tactic:
omega

Result:
no goals

Proof state:
case succ
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
⊢ findGreatest P (k + 1) = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Tactic:
by_cases hk : P (k + 1)

Result:
case pos
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ findGreatest P (k + 1) = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

case neg
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : ¬P (k + 1)
⊢ findGreatest P (k + 1) = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Proof state:
case pos.mp
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ k + 1 = m → m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Tactic:
rintro rfl

Result:
case pos.mp
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
hk : P (k + 1)
⊢ k + 1 ≤ k + 1 ∧ (k + 1 ≠ 0 → P (k + 1)) ∧ ∀ ⦃n : ℕ⦄, k + 1 < n → n ≤ k + 1 → ¬P n

Proof state:
case pos
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ findGreatest P (k + 1) = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Tactic:
rw [findGreatest_eq hk]

Result:
case pos
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : P (k + 1)
⊢ k + 1 = m ↔ m ≤ k + 1 ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n

Proof state:
case neg.mpr.intro.intro
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : ¬P (k + 1)
hle : m ≤ k + 1
hP : m ≠ 0 → P m
hm : ∀ ⦃n : ℕ⦄, m < n → n ≤ k + 1 → ¬P n
⊢ m ≠ k.succ

Tactic:
rintro rfl

Result:
case neg.mpr.intro.intro
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
hk : ¬P (k + 1)
hle : k.succ ≤ k + 1
hP : k.succ ≠ 0 → P k.succ
hm : ∀ ⦃n : ℕ⦄, k.succ < n → n ≤ k + 1 → ¬P n
⊢ False

Proof state:
case neg.mp.intro.intro
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : ¬P (k + 1)
hle✝ : m ≤ k
hP : m ≠ 0 → P m
hm : ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
n : ℕ
hlt : m < n
hle : n ≤ k + 1
⊢ ¬P n

Tactic:
rcases Decidable.eq_or_lt_of_le hle with (rfl | hlt')

Result:
case neg.mp.intro.intro.inl
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : ¬P (k + 1)
hle✝ : m ≤ k
hP : m ≠ 0 → P m
hm : ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
hlt : m < k + 1
hle : k + 1 ≤ k + 1
⊢ ¬P (k + 1)

case neg.mp.intro.intro.inr
P : ℕ → Prop
inst✝ : DecidablePred P
k : ℕ
ihk : ∀ {m : ℕ}, findGreatest P k = m ↔ m ≤ k ∧ (m ≠ 0 → P m) ∧ ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
m : ℕ
hk : ¬P (k + 1)
hle✝ : m ≤ k
hP : m ≠ 0 → P m
hm : ∀ ⦃n : ℕ⦄, m < n → n ≤ k → ¬P n
n : ℕ
hlt : m < n
hle : n ≤ k + 1
hlt' : n < k + 1
⊢ ¬P n