Notation perm_eql s1 s2 := (perm_eq s1 =1 perm_eq s2).Notation perm_eqr s1 s2 := (perm_eq^~ s1 =1 perm_eq^~ s2).

Lemma rot_add_mod m n s : n <= size s -> m <= size s -> rot m (rot n s) = rot (if m + n <= size s then m + n else m + n - size s) s.