Section Cutting.
Variable A : Type.
Fixpoint firstn (n:nat)(l:list A) : list A :=
match n with
| 0 => nil
| S n => match l with
| nil => nil
| a::l => a::(firstn n l)
end
end.
Fixpoint skipn (n:nat)(l:list A) : list A :=
match n with
| 0 => l
| S n => match l with
| nil => nil
| a::l => skipn n l
end
end.

Lemma firstn_skipn : forall n l, firstn n l ++ skipn n l = l.