-- Mathlib/RingTheory/UniqueFactorizationDomain/FactorSet.lean
theorem exists_prime_dvd_of_not_inf_one {a b : α} (ha : a ≠ 0) (hb : b ≠ 0)
    (h : Associates.mk a ⊓ Associates.mk b ≠ 1) : ∃ p : α, Prime p ∧ p ∣ a ∧ p ∣ b

Proof state:
case intro.mk.intro.refine_3
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ p ∣ b

Tactic:
apply dvd_of_mk_le_mk

Result:
case intro.mk.intro.refine_3.a
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ Associates.mk p ≤ Associates.mk b

Proof state:
case intro.mk.intro.refine_1
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ Prime p

Tactic:
rw [← UniqueFactorizationMonoid.irreducible_iff_prime, ← irreducible_mk]

Result:
case intro.mk.intro.refine_1
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ Irreducible (Associates.mk p)

Proof state:
case intro.mk.intro.refine_2.a
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ Associates.mk p ≤ Associates.mk a

Tactic:
apply dvd_of_mem_factors' (Multiset.mem_inter.mp p0_mem).left

Result:
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ a ≠ 0

Proof state:
case intro.mk.intro
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ ∃ p, Prime p ∧ p ∣ a ∧ p ∣ b

Tactic:
refine ⟨p, ?_, ?_, ?_⟩

Result:
case intro.mk.intro.refine_1
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ Prime p

case intro.mk.intro.refine_2
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ p ∣ a

case intro.mk.intro.refine_3
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
hz : ↑(factors' a ⊓ factors' b) ≠ 0
p : α
p0_irr : Irreducible (Associates.mk p)
p0_mem : ⟨Associates.mk p, p0_irr⟩ ∈ factors' a ∩ factors' b
⊢ p ∣ b

Proof state:
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
h : Associates.mk a ⊓ Associates.mk b ≠ 1
⊢ (Associates.mk a).factors ⊓ (Associates.mk b).factors ≠ 0

Tactic:
contrapose! h with hf

Result:
α : Type u_1
inst✝¹ : CancelCommMonoidWithZero α
inst✝ : UniqueFactorizationMonoid α
a b : α
ha : a ≠ 0
hb : b ≠ 0
hf : (Associates.mk a).factors ⊓ (Associates.mk b).factors = 0
⊢ Associates.mk a ⊓ Associates.mk b = 1