Section RotrLemmas.
Variables (n0 : nat) (T : Type) (T' : eqType).
Implicit Types (x : T) (s : seq T).
Definition rot_add s n m (k := size s) (p := minn m k + minn n k) :=
locked (if p <= k then p else p - k).

Lemma subseq_cat2l s s1 s2 : subseq (s ++ s1) (s ++ s2) = subseq s1 s2.