-- Lean 4 proof step from Mathlib.LinearAlgebra.LinearIndependent.Defs (goal)
Goal:
ι : Type u'
R : Type u_2
M : Type u_4
v : ι → M
inst : Semiring R
inst_1 : AddCommMonoid M
inst_2 : Module R M
inst_3 : LinearOrder R
inst_4 : CanonicallyOrderedAdd R
inst_5 : AddRightReflectLE R
inst_6 : IsCancelAdd M
letI letI₁ letI₂ :
  LinearIndependent R v ↔
    ∀ (s t : Finset ι) (f : ι → R),
      Disjoint s t →
        Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ t, f i • v i) → (∀ i ∈ s, f i = 0) ∧ ∀ i ∈ t, f i = 0
letI₃ :
  let this : Sub R := CanonicallyOrderedAdd.toSub;
  ∀ (this : OrderedSub R),
    LinearIndependent R v ↔
      ∀ (s t : Finset ι) (f : ι → R),
        Disjoint s t →
          Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ t, f i • v i) → (∀ i ∈ s, f i = 0) ∧ ∀ i ∈ t, f i = 0
letI₄ :
  let this : Sub R := CanonicallyOrderedAdd.toSub;
  ∀ (this : OrderedSub R),
    (∀ (s : Finset ι) (f g : ι → R),
        Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ s, g i • v i) → ∀ i ∈ s, f i = g i) ↔
      ∀ (s t : Finset ι) (f : ι → R),
        Disjoint s t →
          Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ t, f i • v i) → (∀ i ∈ s, f i = 0) ∧ ∀ i ∈ t, f i = 0
⊢ LinearIndependent R v ↔
    ∀ (s t : Finset ι) (f : ι → R),
      Disjoint s t →
        Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ t, f i • v i) → (∀ i ∈ s, f i = 0) ∧ ∀ i ∈ t, f i = 0
have refine_1 : let this : Sub R := CanonicallyOrderedAdd.toSub;
∀ (this : OrderedSub R)
  (h :
    ∀ (s : Finset ι) (f g : ι → R),
      Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ s, g i • v i) → ∀ i ∈ s, f i = g i)
  (s t : Finset ι) (f : ι → R) (hst : Disjoint s t)
  (heq : Eq.{u_4 + 1} (α := M) (∑ i ∈ s, f i • v i) (∑ i ∈ t, f i • v i)), (∀ i ∈ s, f i = 0) ∧ ∀ i ∈ t, f i = 0
Tactic: classical
  letI : Sub R := CanonicallyOrderedAdd.toSub
  haveI : OrderedSub R := CanonicallyOrderedAdd.toOrderedSub
  rw [linearIndependent_iff'ₛ]
  refine ⟨fun h s t f hst heq => ?_, fun h s f g heq => ?_⟩
  · specialize h (s ∪ t) (fun i => if i ∈ s then f i else 0) (fun i => if i ∈ t then f i else 0) ?_
    · simpa
    refine ⟨fun i hi => ?_, fun i hi => ?_⟩
    · simpa [hi, hst.notMem_of_mem_left_finset hi] using h i (Finset.mem_union_left _ hi)
    · simpa [hi, hst.notMem_of_mem_right_finset hi] using (h i (Finset.mem_union_right _ hi)).symm
  · specialize h { i ∈ s | g i ≤ f i } { i ∈ s | f i < g i }
      (fun i => if g i ≤ f i then f i - g i else g i - f i) ?_ ?_
    · simp_rw [Finset.disjoint_left, Finset.mem_filter]
      exact fun i ⟨_, hi⟩ ⟨_, hi'⟩ => hi.not_gt hi'
    · rw [← add_right_cancel_iff
        (a := ∑ i ∈ s with g i ≤ f i, g i • v i + ∑ i ∈ s with f i < g i, f i • v i)]
      conv_lhs => rw [← add_assoc, ← Finset.sum_add_distrib]
      conv_rhs => rw [add_left_comm, ← Finset.sum_add_distrib]
      convert! heq
        <;> simp_rw [← Finset.sum_filter_add_sum_filter_not s (fun i => g i ≤ f i), not_le]
        <;> congr! 2 with i hi
        <;> simp only [Finset.mem_filter] at hi
      · simp [hi.2, ← add_smul, tsub_add_cancel_of_le hi.2]
      · simp [hi.2.not_ge, ← add_smul, tsub_add_cancel_of_le hi.2.le]
    simp only [Finset.mem_filter] at h
    intro i hi
    by_cases hi' : g i ≤ f i
    · apply hi'.antisymm'
      simpa [hi', tsub_eq_zero_iff_le] using h.1 i ⟨hi, hi'⟩
    · apply (not_le.1 hi').le.antisymm
      simpa [hi', tsub_eq_zero_iff_le] using h.2 i ⟨hi, not_le.1 hi'⟩