(** Relative positions, from greatest to smallest: << max_unsigned max_signed 2*wordsize-1 wordsize 0 min_signed >> *)
Remark half_modulus_pos: half_modulus > 0.
Remark min_signed_neg: min_signed < 0.
Remark max_signed_pos: max_signed >= 0.
Remark wordsize_max_unsigned: zwordsize <= max_unsigned.
Remark two_wordsize_max_unsigned: 2 * zwordsize - 1 <= max_unsigned.
Remark max_signed_unsigned: max_signed < max_unsigned.

(* Specifications: *)
Lemma unsigned_repr_eq:
forall x, unsigned (repr x) = Z.modulo x modulus.
Lemma signed_repr_eq:
forall x, signed (repr x) = if zlt (Z.modulo x modulus) half_modulus then Z.modulo x modulus else Z.modulo x modulus - modulus.