-- Mathlib/LinearAlgebra/Matrix/Transvection.lean
theorem listTransvecCol_mul_last_col (hM : M (inr unit) (inr unit) ≠ 0) (i : Fin r) :
    ((listTransvecCol M).prod * M) (inl i) (inr unit) = 0

Proof state:
case of_succ
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
⊢ (((listTransvecCol M)[n] :: List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) =
    if n ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
have A :
  (listTransvecCol M)[n] =
    transvection (inl n') (inr unit) (-M (inl n') (inr unit) / M (inr unit) (inr unit)) := by
  simp [n', listTransvecCol]

Result:
case of_succ
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
⊢ (((listTransvecCol M)[n] :: List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) =
    if n ≤ ↑i then 0 else M (inl i) (inr ())

Proof state:
case neg
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : ¬n' = i
hni : n ≠ ↑i
⊢ (if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())) = if n ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
rcases le_or_lt (n + 1) i with (hi | hi)

Result:
case neg.inl
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : ¬n' = i
hni : n ≠ ↑i
hi : n + 1 ≤ ↑i
⊢ (if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())) = if n ≤ ↑i then 0 else M (inl i) (inr ())

case neg.inr
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : ¬n' = i
hni : n ≠ ↑i
hi : ↑i < n + 1
⊢ (if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())) = if n ≤ ↑i then 0 else M (inl i) (inr ())

Proof state:
case of_succ
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
⊢ ((List.drop n (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
let n' : Fin r := ⟨n, hn⟩

Result:
case of_succ
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
⊢ ((List.drop n (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n ≤ ↑i then 0 else M (inl i) (inr ())

Proof state:
case mk
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
k n : ℕ
hn : n < r
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
val✝ : ℕ
isLt✝ : val✝ < r
IH :
  ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl ⟨val✝, isLt✝⟩) (inr ()) =
    if n + 1 ≤ ↑⟨val✝, isLt✝⟩ then 0 else M (inl ⟨val✝, isLt✝⟩) (inr ())
h : n = val✝
⊢ n = ↑⟨val✝, isLt✝⟩

Tactic:
simp [h]

Result:
no goals

Proof state:
case self
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k : ℕ
⊢ ((List.drop r (listTransvecCol M)).prod * M) (inl i) (inr ()) = if r ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
simp only [length_listTransvecCol, le_refl, List.drop_eq_nil_of_le, List.prod_nil,
  Matrix.one_mul]

Result:
case self
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k : ℕ
⊢ M (inl i) (inr ()) = if r ≤ ↑i then 0 else M (inl i) (inr ())

Proof state:
case pos
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : n' = i
⊢ (transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ())) *
        ((List.drop (n + 1) (listTransvecCol M)).prod * M))
      (inl i) (inr ()) =
    if n ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
have hni : n = i := by
  cases i
  simp only [n', Fin.mk_eq_mk] at h
  simp [h]

Result:
case pos
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : n' = i
hni : n = ↑i
⊢ (transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ())) *
        ((List.drop (n + 1) (listTransvecCol M)).prod * M))
      (inl i) (inr ()) =
    if n ≤ ↑i then 0 else M (inl i) (inr ())

Proof state:
case neg
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : ¬n' = i
hni : n ≠ ↑i
⊢ ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n ≤ ↑i then 0 else M (inl i) (inr ())

Tactic:
rw [IH]

Result:
case neg
𝕜 : Type u_3
inst✝ : Field 𝕜
r : ℕ
M : Matrix (Fin r ⊕ Unit) (Fin r ⊕ Unit) 𝕜
hM : M (inr ()) (inr ()) ≠ 0
i : Fin r
k n : ℕ
hn : n < r
IH : ((List.drop (n + 1) (listTransvecCol M)).prod * M) (inl i) (inr ()) = if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())
hn' : n < (listTransvecCol M).length
n' : Fin r := ⟨n, hn⟩
A : (listTransvecCol M)[n] = transvection (inl n') (inr ()) (-M (inl n') (inr ()) / M (inr ()) (inr ()))
h : ¬n' = i
hni : n ≠ ↑i
⊢ (if n + 1 ≤ ↑i then 0 else M (inl i) (inr ())) = if n ≤ ↑i then 0 else M (inl i) (inr ())