case nil => 
  simp [mul]
  rfl
R : Type
inst✝³ : Ring R
inst✝² : DecidableEq R
G : Type
inst✝¹ : Group G
inst✝ : DecidableEq G
s₁ s₂ : FormalSum R G
rel : ElementaryMove R G s₁ s₂
⊢ mul s₁ [] ≈ mul s₂ []
no goals