(** ** Properties of negation *)

(* Specifications: *)
Theorem neg_repr: forall z, neg (repr z) = repr (-z).
Theorem neg_zero: neg zero = zero.
Theorem neg_involutive: forall x, neg (neg x) = x.
Theorem neg_add_distr: forall x y, neg(add x y) = add (neg x) (neg y).
(** ** Properties of subtraction *)
Theorem sub_zero_l: forall x, sub x zero = x.
Theorem sub_zero_r: forall x, sub zero x = neg x.
Theorem sub_add_opp: forall x y, sub x y = add x (neg y).
Theorem sub_idem: forall x, sub x x = zero.
Theorem sub_add_l: forall x y z, sub (add x y) z = add (sub x z) y.
Theorem sub_add_r: forall x y z, sub x (add y z) = add (sub x z) (neg y).
Theorem sub_shifted:
forall x y z,
sub (add x z) (add y z) = sub x y.
Theorem sub_signed:
forall x y, sub x y = repr (signed x - signed y).
Theorem unsigned_sub_borrow:
forall x y,
unsigned (sub x y) = unsigned x - unsigned y + unsigned (sub_borrow x y zero) * modulus.