End Concat.
Section Incl.

(* Specifications: *)
Lemma incl_nil {A:Type} (l:list A): incl [] l.
Lemma incl_flat_map_incl {A B C:Type} (xs:list A) (r:A→list B) (rh:B→list C) lh:
(∀ x : A, x ∈ xs → incl (flat_map rh (r x)) (lh x)) →
incl (flat_map rh (concat (map r xs))) (flat_map lh xs).