Section Reverse_Induction.
End Reverse_Induction.
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(** ** Concatenation     *)
(*************************)
Fixpoint concat (l : list (list A)) : list A :=
match l with
| nil => nil
| cons x l => x ++ concat l
end.

Lemma concat_app : forall l1 l2, concat (l1 ++ l2) = concat l1 ++ concat l2.