-- Mathlib/.lake/packages/lean4/src/lean/Std/Tactic/BVDecide/LRAT/Internal/Formula/RatAddSound.lean
theorem ratAdd_sound {n : Nat} (f : DefaultFormula n) (c : DefaultClause n)
    (pivot : Literal (PosFin n)) (rupHints : Array Nat) (ratHints : Array (Nat × Array Nat))
    (f' : DefaultFormula n) (f_readyForRatAdd : ReadyForRatAdd f) (pivot_in_c : pivot ∈ Clause.toList c)
    (ratAddSuccess : performRatAdd f c pivot rupHints ratHints = (f', true)) :
    Equisat (PosFin n) f f'

Proof state:
n : Nat
f : DefaultFormula n
c : DefaultClause n
pivot : Literal (PosFin n)
rupHints : Array Nat
ratHints : Array (Nat × Array Nat)
f' : DefaultFormula n
f_readyForRatAdd : f.ReadyForRatAdd
pivot_in_c : pivot ∈ Clause.toList c
f'_def : f' = f.insert c
ratHintsExhaustive_eq_true : f.ratHintsExhaustive pivot ratHints = true
ratAddSuccess :
  (if (f.insertRupUnits c.negate).snd = true then ((f.insertRupUnits c.negate).fst, false)
    else
      if ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.2.snd = true then
        (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, false)
      else
        if ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.2.fst = true then
          (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, false)
        else
          if
              (Array.foldl
                    (fun x ratHint =>
                      if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                    (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).snd =
                false then
            ((Array.foldl
                  (fun x ratHint => if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                  (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).fst,
              false)
          else
            ({
                      clauses :=
                        (Array.foldl
                              (fun x ratHint =>
                                if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                              (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true)
                              ratHints).1.clauses,
                      rupUnits :=
                        (Array.foldl
                              (fun x ratHint =>
                                if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                              (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true)
                              ratHints).1.rupUnits,
                      ratUnits :=
                        (Array.foldl
                              (fun x ratHint =>
                                if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                              (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true)
                              ratHints).1.ratUnits,
                      assignments :=
                        restoreAssignments
                          (Array.foldl
                                (fun x ratHint =>
                                  if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                                (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true)
                                ratHints).1.assignments
                          ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.fst }.clearRupUnits.insert
                c,
              true)) =
    (f', true)
⊢ Equisat (PosFin n) f f'

Tactic:
split at ratAddSuccess

Result:
case isTrue
n : Nat
f : DefaultFormula n
c : DefaultClause n
pivot : Literal (PosFin n)
rupHints : Array Nat
ratHints : Array (Nat × Array Nat)
f' : DefaultFormula n
f_readyForRatAdd : f.ReadyForRatAdd
pivot_in_c : pivot ∈ Clause.toList c
f'_def : f' = f.insert c
ratHintsExhaustive_eq_true : f.ratHintsExhaustive pivot ratHints = true
h✝ : (f.insertRupUnits c.negate).snd = true
ratAddSuccess : ((f.insertRupUnits c.negate).fst, false) = (f', true)
⊢ Equisat (PosFin n) f f'

case isFalse
n : Nat
f : DefaultFormula n
c : DefaultClause n
pivot : Literal (PosFin n)
rupHints : Array Nat
ratHints : Array (Nat × Array Nat)
f' : DefaultFormula n
f_readyForRatAdd : f.ReadyForRatAdd
pivot_in_c : pivot ∈ Clause.toList c
f'_def : f' = f.insert c
ratHintsExhaustive_eq_true : f.ratHintsExhaustive pivot ratHints = true
h✝ : ¬(f.insertRupUnits c.negate).snd = true
ratAddSuccess :
  (if ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.2.snd = true then
      (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, false)
    else
      if ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.2.fst = true then
        (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, false)
      else
        if
            (Array.foldl
                  (fun x ratHint => if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                  (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).snd =
              false then
          ((Array.foldl
                (fun x ratHint => if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).fst,
            false)
        else
          ({
                    clauses :=
                      (Array.foldl
                            (fun x ratHint =>
                              if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                            (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).1.clauses,
                    rupUnits :=
                      (Array.foldl
                            (fun x ratHint =>
                              if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                            (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).1.rupUnits,
                    ratUnits :=
                      (Array.foldl
                            (fun x ratHint =>
                              if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                            (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true) ratHints).1.ratUnits,
                    assignments :=
                      restoreAssignments
                        (Array.foldl
                              (fun x ratHint =>
                                if x.snd = true then x.fst.performRatCheck pivot.negate ratHint else (x.fst, false))
                              (((f.insertRupUnits c.negate).fst.performRupCheck rupHints).fst, true)
                              ratHints).1.assignments
                        ((f.insertRupUnits c.negate).fst.performRupCheck rupHints).2.fst }.clearRupUnits.insert
              c,
            true)) =
    (f', true)
⊢ Equisat (PosFin n) f f'