Section SetIncl.
Variable A : Type.
Definition incl (l m:list A) := forall a:A, In a l -> In a m.
Hint Unfold incl : core.
Hint Resolve incl_refl : core.
Hint Immediate incl_tl : core.
Hint Immediate incl_appl : core.

Lemma incl_appr : forall l m n:list A, incl l n -> incl l (m ++ n).