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 cat_subseq s1 s2 s3 s4 :
subseq s1 s3 -> subseq s2 s4 -> subseq (s1 ++ s2) (s3 ++ s4).