∀ {V : Type u_1} [inst : NormedAddCommGroup V] [inst_1 : InnerProductSpace ℝ V] [hd2 : Fact (Module.finrank ℝ V = 2)]
  (o : Orientation ℝ V (Fin 2)) {x y : V},
  o.oangle x y = ↑(Real.pi / 2) → o.oangle (x - y) x = ↑(Real.arctan (‖y‖ / ‖x‖))