. split
  . exact h3
  . simp only
    by_cases i.1 = l.1.1
    . next i_eq_l =>
      simp only [i_eq_l]
      rw [Array.get_modify_at_idx l_in_bounds]
      simp only [← i_eq_l, h3, add_of_both_eq_both]
    . next i_ne_l => rw [Array.get_modify_unchanged l_in_bounds i_in_bounds _ (Ne.symm i_ne_l), h3]
case inr.inr.intro.intro.intro.intro.intro.intro.intro.right.right.left
n : Nat
assignments0 : Array Assignment
assignments0_size : assignments0.size = n
units : Array (Literal (PosFin n))
assignments : Array Assignment
assignments_size : assignments.size = n
foundContradiction : Bool
l : Literal (PosFin n)
i : Fin n
i_in_bounds : ↑i < assignments.size
l_in_bounds : l.fst.val < assignments.size
j1 j2 : Fin units.size
i_gt_zero : ↑i > 0
h1 : units[↑j1] = (⟨↑i, ⋯⟩, true)
h2 : units[↑j2] = (⟨↑i, ⋯⟩, false)
h3 : assignments[↑i] = both
h4 : assignments0[↑i] = unassigned
h5 : ∀ (k : Fin units.size), ¬k = j1 → ¬k = j2 → ¬units[↑k].fst.val = ↑i
j1_lt_updatedUnits_size : ↑j1 < (insertUnit (units, assignments, foundContradiction) l).fst.size
j2_lt_updatedUnits_size : ↑j2 < (insertUnit (units, assignments, foundContradiction) l).fst.size
⊢ (if hasAssignment l.snd assignments[l.fst.val]! = true then (units, assignments, foundContradiction)
          else
            (units.push l, assignments.modify l.fst.val (addAssignment l.snd),
              foundContradiction || assignments[l.fst.val]! != unassigned)).snd.fst[↑i] =
    both

case inr.inr.intro.intro.intro.intro.intro.intro.intro.right.right.right
n : Nat
assignments0 : Array Assignment
assignments0_size : assignments0.size = n
units : Array (Literal (PosFin n))
assignments : Array Assignment
assignments_size : assignments.size = n
foundContradiction : Bool
l : Literal (PosFin n)
i : Fin n
i_in_bounds : ↑i < assignments.size
l_in_bounds : l.fst.val < assignments.size
j1 j2 : Fin units.size
i_gt_zero : ↑i > 0
h1 : units[↑j1] = (⟨↑i, ⋯⟩, true)
h2 : units[↑j2] = (⟨↑i, ⋯⟩, false)
h3 : assignments[↑i] = both
h4 : assignments0[↑i] = unassigned
h5 : ∀ (k : Fin units.size), ¬k = j1 → ¬k = j2 → ¬units[↑k].fst.val = ↑i
j1_lt_updatedUnits_size : ↑j1 < (insertUnit (units, assignments, foundContradiction) l).fst.size
j2_lt_updatedUnits_size : ↑j2 < (insertUnit (units, assignments, foundContradiction) l).fst.size
⊢ assignments0[↑i] = unassigned ∧
    ∀
      (k :
        Fin
          (if hasAssignment l.snd assignments[l.fst.val]! = true then (units, assignments, foundContradiction)
              else
                (units.push (l.fst, l.snd), assignments.modify l.fst.val (addAssignment l.snd),
                  foundContradiction || assignments[l.fst.val]! != unassigned)).fst.size),
      ¬k = ⟨↑j1, j1_lt_updatedUnits_size⟩ →
        ¬k = ⟨↑j2, j2_lt_updatedUnits_size⟩ →
          ¬(if hasAssignment l.snd assignments[l.fst.val]! = true then (units, assignments, foundContradiction)
                      else
                        (units.push l, assignments.modify l.fst.val (addAssignment l.snd),
                          foundContradiction || assignments[l.fst.val]! != unassigned)).fst[↑k].fst.val =
              ↑i
case inr.inr.intro.intro.intro.intro.intro.intro.intro.right.right.right
n : Nat
assignments0 : Array Assignment
assignments0_size : assignments0.size = n
units : Array (Literal (PosFin n))
assignments : Array Assignment
assignments_size : assignments.size = n
foundContradiction : Bool
l : Literal (PosFin n)
i : Fin n
i_in_bounds : ↑i < assignments.size
l_in_bounds : l.fst.val < assignments.size
j1 j2 : Fin units.size
i_gt_zero : ↑i > 0
h1 : units[↑j1] = (⟨↑i, ⋯⟩, true)
h2 : units[↑j2] = (⟨↑i, ⋯⟩, false)
h3 : assignments[↑i] = both
h4 : assignments0[↑i] = unassigned
h5 : ∀ (k : Fin units.size), ¬k = j1 → ¬k = j2 → ¬units[↑k].fst.val = ↑i
j1_lt_updatedUnits_size : ↑j1 < (insertUnit (units, assignments, foundContradiction) l).fst.size
j2_lt_updatedUnits_size : ↑j2 < (insertUnit (units, assignments, foundContradiction) l).fst.size
⊢ assignments0[↑i] = unassigned ∧
    ∀
      (k :
        Fin
          (if hasAssignment l.snd assignments[l.fst.val]! = true then (units, assignments, foundContradiction)
              else
                (units.push (l.fst, l.snd), assignments.modify l.fst.val (addAssignment l.snd),
                  foundContradiction || assignments[l.fst.val]! != unassigned)).fst.size),
      ¬k = ⟨↑j1, j1_lt_updatedUnits_size⟩ →
        ¬k = ⟨↑j2, j2_lt_updatedUnits_size⟩ →
          ¬(if hasAssignment l.snd assignments[l.fst.val]! = true then (units, assignments, foundContradiction)
                      else
                        (units.push l, assignments.modify l.fst.val (addAssignment l.snd),
                          foundContradiction || assignments[l.fst.val]! != unassigned)).fst[↑k].fst.val =
              ↑i