-- Mathlib/LinearAlgebra/Basis/Basic.lean
theorem maximal [Nontrivial R] (b : Basis ι R M) : b.linearIndependent.Maximal := fun w hi h => by
  -- If `w` is strictly bigger than `range b`,
  apply le_antisymm h
  -- then choose some `x ∈ w \ range b`,
  intro x p
  by_contra q
  -- and write it in terms of the basis.
  have e := b.linearCombination_repr x
  -- This then expresses `x` as a linear combination
  -- of elements of `w` which are in the range of `b`,
  let u : ι ↪ w :=
    ⟨fun i => ⟨b i, h ⟨i, rfl⟩⟩, fun i i' r =>
      b.injective (by simpa only [Subtype.mk_eq_mk] using r)⟩
  simp_rw [Finsupp.linearCombination_apply] at e
  change ((b.repr x).sum fun (i : ι) (a : R) ↦ a • (u i : M)) = ((⟨x, p⟩ : w) : M) at e
  rw [← Finsupp.sum_embDomain (f := u) (g := fun x r ↦ r • (x : M)),
      ← Finsupp.linearCombination_apply] at e
  -- Now we can contradict the linear independence of `hi`
  refine hi.linearCombination_ne_of_not_mem_support _ ?_ e
  simp only [Finset.mem_map, Finsupp.support_embDomain]
  rintro ⟨j, -, W⟩
  simp only [u, Embedding.coeFn_mk, Subtype.mk_eq_mk] at W
  apply q ⟨j, W⟩

Proof state:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
⊢ x ∈ range ⇑b

Tactic:
by_contra q

Result:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
⊢ False

Proof state:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
u : ι ↪ ↑w := { toFun := fun i => ⟨b i, ⋯⟩, inj' := ⋯ }
e : (linearCombination R Subtype.val) (embDomain u (b.repr x)) = ↑⟨x, p⟩
⊢ False

Tactic:
refine hi.linearCombination_ne_of_not_mem_support _ ?_ e

Result:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
u : ι ↪ ↑w := { toFun := fun i => ⟨b i, ⋯⟩, inj' := ⋯ }
e : (linearCombination R Subtype.val) (embDomain u (b.repr x)) = ↑⟨x, p⟩
⊢ ⟨x, p⟩ ∉ (embDomain u (b.repr x)).support

Proof state:
case intro.intro
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
u : ι ↪ ↑w := { toFun := fun i => ⟨b i, ⋯⟩, inj' := ⋯ }
e : (linearCombination R Subtype.val) (embDomain u (b.repr x)) = ↑⟨x, p⟩
j : ι
W : b j = x
⊢ False

Tactic:
apply q ⟨j, W⟩

Result:
no goals

Proof state:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
e : (linearCombination R ⇑b) (b.repr x) = x
i i' : ι
r : (fun i => ⟨b i, ⋯⟩) i = (fun i => ⟨b i, ⋯⟩) i'
⊢ b i = b i'

Tactic:
simpa only [Subtype.mk_eq_mk] using r

Result:
no goals

Proof state:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
u : ι ↪ ↑w := { toFun := fun i => ⟨b i, ⋯⟩, inj' := ⋯ }
e : ((b.repr x).sum fun i a => a • ↑(u i)) = ↑⟨x, p⟩
⊢ False

Tactic:
rw [← Finsupp.sum_embDomain (f := u) (g := fun x r ↦ r • (x : M)),
    ← Finsupp.linearCombination_apply] at e

Result:
ι : Type u_1
R : Type u_3
M : Type u_5
inst✝³ : Semiring R
inst✝² : AddCommMonoid M
inst✝¹ : Module R M
inst✝ : Nontrivial R
b : Basis ι R M
w : Set M
hi : LinearIndependent R Subtype.val
h : range ⇑b ≤ w
x : M
p : x ∈ w
q : x ∉ range ⇑b
u : ι ↪ ↑w := { toFun := fun i => ⟨b i, ⋯⟩, inj' := ⋯ }
e : (linearCombination R Subtype.val) (embDomain u (b.repr x)) = ↑⟨x, p⟩
⊢ False