Proof (same_bits_eqmod wordsize).
Remark two_power_nat_infinity:
forall x, 0 <= x -> exists n, x < two_power_nat n.

Lemma Ztestbit_above_neg:
forall n x i,
-two_power_nat n <= x < 0 ->
i >= Z.of_nat n ->
Z.testbit x i = true.