(* A_1 on yk: *)

Lemma A_1_B_1_B_yk : forall(m:fmap)(k:dim)(x0 y0:dart),
inv_qhmap m ->
exd m x0 -> exd m y0 ->
let xk := A m k x0 in
let yk := A_1 m k y0 in
y0 <> yk -> y0 <> xk ->
A_1 (B_1 (B m k x0) k y0) k yk =
if eq_dart_dec xk yk then nil
else A_1 m k yk.