(* A_1 on xk: *)

(* Specifications: *)
Lemma A_1_B_1_B_xk : forall(m:fmap)(k:dim)(x0 y0:dart),
inv_qhmap m ->
exd m x0 -> exd m y0 -> y0<>A m k x0 ->
A_1 (B_1 (B m k x0) k y0) k (A m k x0) = nil.