(** * Properties of shifts *)
(** First, a unified specification for [shiftl] : the [shiftl_spec] below (combined with [testbit_neg_r]) is equivalent to [shiftl_spec_low] and [shiftl_spec_high]. *)

(** A shiftl by a negative number is a shiftr, and vice-versa *)
Lemma shiftr_opp_r : forall a n, a >> (-n) == a << n.