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.
Hint Immediate incl_appr : core.

Lemma incl_cons :
forall (a:A) (l m:list A), In a m -> incl l m -> incl (a :: l) m.