Section Fprop_Sterbenz.
Variable beta : radix.
Notation bpow e := (bpow beta e).
Variable fexp : Z -> Z.
Context { valid_exp : Valid_exp fexp }.
Context { monotone_exp : Monotone_exp fexp }.
Notation format := (generic_format beta fexp).

Theorem sterbenz :
forall x y, format x -> format y ->
(y / 2 <= x <= 2 * y)%R ->
format (x - y)%R.