proposition: Lemma rotrK : cancel (@rotr T n0) (rot n0).
proof: Proof. move=> s; have [lt_n0s | ge_n0s] := ltnP n0 (size s). by rewrite -{1}(subKn (ltnW lt_n0s)) -{1}[size s]size_rotr; apply: rotK. by rewrite -[in RHS](rot_oversize ge_n0s) /rotr (eqnP ge_n0s) rot0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)