(* COROLLARY, OK!: *)

(* Specifications: *)
Theorem nf_B0:forall (m:fmap)(x:dart),
inv_hmap m -> succ m zero x ->
let y := A m zero x in
let x0 := bottom m zero x in
nf (B m zero x) = nf m +
if expf_dec m y x0 then 1 else -1.