Definition shorter_ss_l (ss1 ss2 : sseq) (o i: nat) : Prop :=
forall (j : nat), j <= i-o -> ss1.[i-j] = ss2.[i-j].
Definition shorter_ss_u (ss1 ss2 : sseq) (i d: nat) : Prop :=
forall (j : nat), ss1.[i+j+d] = ss2.[i+j].
Definition shorter_ss (ss1 ss2 : sseq) (o i d: nat) : Prop :=
shorter_ss_l ss1 ss2 o i /\ shorter_ss_u ss1 ss2 i d.

Lemma skipn_shorter_ss_u :
forall (i j d : nat) (ss1 ss2 : sseq),
shorter_ss_u ss1 ss2 (i+j) d <->
shorter_ss_u (skipn i ss1) (skipn i ss2) j d.