∀ (i : String.Pos.Raw) (c : Char) (cs : List Char) (a : String.Pos.Raw) (b₁ b₂ : ℕ),
  (String.Pos.Raw.utf8GetAux cs a i).utf8Size + b₁ = String.utf8ByteSize'.go✝ cs + b₂ →
    (String.Pos.Raw.utf8GetAux (String.Pos.Raw.utf8SetAux c cs a i) a i).utf8Size + b₁ =
      String.utf8ByteSize'.go✝¹ (String.Pos.Raw.utf8SetAux c cs a i) + b₂