Canonical Structure search_right x (h : heap) (f : find x) :=
@Form x (right_tag (h :+ untag (heap_of f))) (@right_pf x h f).

(* Main lemma *)
Lemma indom (x : ptr) (f : find x) : def f -> x \in dom f.
Example ex2 A (x1 x2 : ptr) (v1 v2 : A) (h1 h2 : heap) :
def (h1 :+ x1 :-> 1 :+ (x2 :-> 3 :+ empty)) ->
if x2 \in dom (h1 :+ x1 :-> 1 :+ (x2 :-> 3 :+ empty))
then 1 == 1
else 1 == 0.