(* THE SYMMETRICS, PROVEN TOO: *)
(* A_1 on y0: *)

(* Specifications: *)
Lemma A_1_B_1_B_y0 : forall(m:fmap)(k:dim)(x0 y0:dart),        inv_qhmap m ->
A_1 (B_1 (B m k x0) k y0) k y0 = nil.