(* OK: *)

Theorem eqc_B: forall(m:fmap)(k:dim)(x z t:dart),
inv_hmap m -> succ m k x ->
let xk:= A m k x in
let m0:= B m k x in
(eqc m z t <->
(eqc m0 z t \/
eqc m0 z x /\ eqc m0 xk t \/
eqc m0 z xk /\ eqc m0 x t)).