Fixpoint nsum (n:Nat) {struct n} : seq n Nat -> Nat :=
match n return (seq n Nat -> Nat) with
|O => fun x:seq 0 Nat => 0
|S m => fun x:seq (S m) Nat => (head x) + (nsum m (Seqtl x)) end.

Lemma seqtlobv:forall(n:Nat)(G:Setoid)(A:part_set G) (AH:seq(S n)A) (g:G)(gH:in_part g (seq_set(Seqtl(Map_embed AH)))), in_part g A.