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 In_inv : forall k k' e l, In k ((k',e) :: l) -> eq k k' \/ In k l.