-- Mathlib/CategoryTheory/Abelian/Projective/Dimension.lean
lemma hasProjectiveDimensionLT_X₁ (h₂ : HasProjectiveDimensionLT S.X₂ n)
    (h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)) :
    HasProjectiveDimensionLT S.X₁ n

Proof state:
case intro
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₂ : Ext S.X₂ Y i
⊢ (Ext.mk₀ S.f).comp x₂ ⋯ = 0

Tactic:
rw [x₂.eq_zero_of_hasProjectiveDimensionLT n (by omega), Ext.comp_zero]

Result:
no goals

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₁ : Ext S.X₁ Y i
⊢ n + 1 ≤ i + 1

Tactic:
omega

Result:
no goals

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
⊢ ∀ (i : ℕ), n ≤ i → ∀ ⦃Y : C⦄ (e : Ext S.X₁ Y i), e = 0

Tactic:
intro i hi Y x₁

Result:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₁ : Ext S.X₁ Y i
⊢ x₁ = 0

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₂ : Ext S.X₂ Y i
⊢ n ≤ i

Tactic:
omega

Result:
no goals

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
⊢ HasProjectiveDimensionLT S.X₁ n

Tactic:
letI := HasExt.standard C

Result:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
⊢ HasProjectiveDimensionLT S.X₁ n

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₁ : Ext S.X₁ Y i
⊢ x₁ = 0

Tactic:
obtain ⟨x₂, rfl⟩ := Ext.contravariant_sequence_exact₁ hS _ x₁ (add_comm _ _)
  (Ext.eq_zero_of_hasProjectiveDimensionLT _ (n + 1) (by omega))

Result:
case intro
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
i : ℕ
hi : n ≤ i
Y : C
x₂ : Ext S.X₂ Y i
⊢ (Ext.mk₀ S.f).comp x₂ ⋯ = 0

Proof state:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
⊢ HasProjectiveDimensionLT S.X₁ n

Tactic:
rw [hasProjectiveDimensionLT_iff]

Result:
C : Type u
inst✝¹ : Category.{v, u} C
inst✝ : Abelian C
S : ShortComplex C
hS : S.ShortExact
n : ℕ
h₂ : HasProjectiveDimensionLT S.X₂ n
h₃ : HasProjectiveDimensionLT S.X₃ (n + 1)
this : HasExt C := HasExt.standard C
⊢ ∀ (i : ℕ), n ≤ i → ∀ ⦃Y : C⦄ (e : Ext S.X₁ Y i), e = 0