∀ {ι : Type u_1} {M : ι → Type u_2} [inst : (i : ι) → Monoid (M i)] (x x_1 j k : ι) (_hne : j ≠ k)
  (w₁ : Monoid.CoprodI.NeWord M x j) (w₂ : Monoid.CoprodI.NeWord M k x_1), (w₁.append _hne w₂).head = w₁.head