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(** * Cutting a list at some position *)
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Section Cutting.
Variable A : Type.
Local Notation firstn := (@firstn A).
Local Notation skipn := (@skipn A).

Lemma skipn_app n : forall l1 l2,
skipn n (l1 ++ l2) = (skipn n l1) ++ (skipn (n - length l1) l2).