-- Mathlib/SetTheory/Ordinal/Arithmetic.lean
theorem mul_le_of_limit_aux {α β r s} [IsWellOrder α r] [IsWellOrder β s] {c}
    (h : IsLimit (type s)) (H : ∀ b' < type s, type r * b' ≤ c) (l : c < type r * type s) :
    False

Proof state:
case neg
α β : Type u_4
r : α → α → Prop
s : β → β → Prop
inst✝¹ : IsWellOrder α r
inst✝ : IsWellOrder β s
c : Ordinal.{u_4}
h✝ : (type s).IsLimit
H : ∀ b' < type s, type r * b' ≤ c
l : c < type r * type s
a : α
b : β
this✝ : type r * (typein s).toRelEmbedding b + type r ≤ c
this : type r * (typein s).toRelEmbedding b + (typein r).toRelEmbedding a < c
b₁ : β
a₁ : α
h₁ : Prod.Lex s r (b₁, a₁) (b, a)
b₂ : β
a₂ : α
h₂ : Prod.Lex s r (b₂, a₂) (b, a)
h : Subrel (Prod.Lex s r) (fun x => Prod.Lex s r x (b, a)) ⟨(b₁, a₁), h₁⟩ ⟨(b₂, a₂), h₂⟩
e₁ : b = b₁
e₂ : ¬b = b₂
⊢ Sum.Lex (Prod.Lex (Subrel s fun x => s x b) r) (Subrel r fun x => r x a)
    ((fun a_1 =>
        Subtype.casesOn a_1 fun val h =>
          Prod.casesOn (motive := fun x => Prod.Lex s r x (b, a) → { x // s x b } × α ⊕ { x // r x a }) val
            (fun b' a' h => if e : b = b' then Sum.inr ⟨a', ⋯⟩ else Sum.inl (⟨b', ⋯⟩, a')) h)
      ⟨(b₁, a₁), h₁⟩)
    ((fun a_1 =>
        Subtype.casesOn a_1 fun val h =>
          Prod.casesOn (motive := fun x => Prod.Lex s r x (b, a) → { x // s x b } × α ⊕ { x // r x a }) val
            (fun b' a' h => if e : b = b' then Sum.inr ⟨a', ⋯⟩ else Sum.inl (⟨b', ⋯⟩, a')) h)
      ⟨(b₂, a₂), h₂⟩)

Tactic:
subst b₁

Result:
case neg
α β : Type u_4
r : α → α → Prop
s : β → β → Prop
inst✝¹ : IsWellOrder α r
inst✝ : IsWellOrder β s
c : Ordinal.{u_4}
h✝ : (type s).IsLimit
H : ∀ b' < type s, type r * b' ≤ c
l : c < type r * type s
a : α
b : β
this✝ : type r * (typein s).toRelEmbedding b + type r ≤ c
this : type r * (typein s).toRelEmbedding b + (typein r).toRelEmbedding a < c
a₁ : α
b₂ : β
a₂ : α
h₂ : Prod.Lex s r (b₂, a₂) (b, a)
e₂ : ¬b = b₂
h₁ : Prod.Lex s r (b, a₁) (b, a)
h : Subrel (Prod.Lex s r) (fun x => Prod.Lex s r x (b, a)) ⟨(b, a₁), h₁⟩ ⟨(b₂, a₂), h₂⟩
⊢ Sum.Lex (Prod.Lex (Subrel s fun x => s x b) r) (Subrel r fun x => r x a)
    ((fun a_1 =>
        Subtype.casesOn a_1 fun val h =>
          Prod.casesOn (motive := fun x => Prod.Lex s r x (b, a) → { x // s x b } × α ⊕ { x // r x a }) val
            (fun b' a' h => if e : b = b' then Sum.inr ⟨a', ⋯⟩ else Sum.inl (⟨b', ⋯⟩, a')) h)
      ⟨(b, a₁), h₁⟩)
    ((fun a_1 =>
        Subtype.casesOn a_1 fun val h =>
          Prod.casesOn (motive := fun x => Prod.Lex s r x (b, a) → { x // s x b } × α ⊕ { x // r x a }) val
            (fun b' a' h => if e : b = b' then Sum.inr ⟨a', ⋯⟩ else Sum.inl (⟨b', ⋯⟩, a')) h)
      ⟨(b₂, a₂), h₂⟩)

Proof state:
α β : Type u_4
r : α → α → Prop
s : β → β → Prop
inst✝¹ : IsWellOrder α r
inst✝ : IsWellOrder β s
c : Ordinal.{u_4}
h : (type s).IsLimit
H : ∀ b' < type s, type r * b' ≤ c
l : c < type r * type s
a : α
b : β
⊢ (typein (Prod.Lex s r)).toRelEmbedding (b, a) < c

Tactic:
have := H _ (h.succ_lt (typein_lt_type s b))

Result:
α β : Type u_4
r : α → α → Prop
s : β → β → Prop
inst✝¹ : IsWellOrder α r
inst✝ : IsWellOrder β s
c : Ordinal.{u_4}
h : (type s).IsLimit
H : ∀ b' < type s, type r * b' ≤ c
l : c < type r * type s
a : α
b : β
this : type r * succ ((typein s).toRelEmbedding b) ≤ c
⊢ (typein (Prod.Lex s r)).toRelEmbedding (b, a) < c