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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 firstn_app_2 n:
forall l1 l2,
firstn ((length l1) + n) (l1 ++ l2) = l1 ++ firstn n l2.