Definition In k m := exists e:elt, MapsTo k e m.
#[local]
Hint Unfold MapsTo In : core.
(* An alternative formulation for [In k l] is [exists e, InA eqk (k,e) l] *)

Lemma MapsTo_eq : forall l x y e, eq x y -> MapsTo x e l -> MapsTo y e l.