Section Reverse_Induction.
End Reverse_Induction.
(*************************)
(** ** Concatenation     *)
(*************************)
Fixpoint concat (l : list (list A)) : list A :=
match l with
| nil => nil
| cons x l => x ++ concat l
end.
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(** ** Decidable equality on lists *)
(***********************************)
Hypothesis eq_dec : forall (x y : A), {x = y}+{x <> y}.
(** [map] and count of occurrences *)
Hypothesis decA: forall x1 x2 : A, {x1 = x2} + {x1 <> x2}.
Hypothesis decB: forall y1 y2 : B, {y1 = y2} + {y1 <> y2}.
Hypothesis Hfinjective: forall x1 x2: A, (f x1) = (f x2) -> x1 = x2.
(** [flat_map] *)
Definition flat_map (f:A -> list B) :=
fix flat_map (l:list A) : list B :=
match l with
| nil => nil
| cons x t => (f x)++(flat_map t)
end.
Proof using A B.
clear Hfinjective.
induction l; simpl; split; intros.
contradiction.
destruct H as (x,(H,_)); contradiction.
destruct (in_app_or _ _ _ H).
exists a; auto.
destruct (IHl y) as (H1,_); destruct (H1 H0) as (x,(H2,H3)).
exists x; auto.
apply in_or_app.
destruct H as (x,(H0,H1)); destruct H0.
subst; auto.
right; destruct (IHl y) as (_,H2); apply H2.
exists x; auto.
Qed.
End Map.

Lemma concat_map : forall A B (f : A -> B) l, map f (concat l) = concat (map (map f) l).