(** Compatibility with subtraction *)
Remark Zfloor_minus:
forall x n, Zfloor (x - Z2R n) = Zfloor x - n.

Theorem ZofB_minus:
forall minus_nan m f p q,
ZofB f = Some p -> 0 <= p < 2*q -> q <= 2^prec -> (Z2R q <= B2R _ _ f)%R ->
ZofB (Bminus _ _ _ Hmax minus_nan m f (BofZ q)) = Some (p - q).