match hf₂ with
| .nil => 
  constructor
| .cons (t := t) (ts := ts) lt _ _ => 
  rename_i hneq
  rw[hrank_of_cons] at *
  constructor
  . rw[← heq] at lt
    unfold bne at hneq
    rw[Nat.not_beq_eq_true_eq] at hneq
    apply lt_of_le_of_ne <;> assumption
  repeat assumption
case a.cons
α : Type u_1
le : α → α → Bool
r : ℕ
xs ys : List (BinTree α)
h₁ : BinTree α
t₁ : List (BinTree α)
h₂ : BinTree α
t₂ : List (BinTree α)
ht₁ : IsHeapForest' r (h₁ :: t₁)
h✝³ : (h₁.rank + 1 != hRank t₂) = true
hf : IsHeapForest' r t₁
not_le_le : ∀ (x y : α), le x y = false → le y x = true
h✝² : h₂.rank ≤ h₁.rank
h✝¹ : h₁.rank ≤ h₂.rank
h✝ : (h₁.rank + 1 != hRank t₁) = false
heq : h₁.rank = h₂.rank
a✝¹ : IsBinTree h₂
a✝ : r < h₂.rank
hf₂ : IsHeapForest' h₂.rank t₂
⊢ IsHeapForest' (h₁.rank + 1) t₂
no goals