Definition lfresh h := (last null (get_lows h)) .+ 2.
Definition hfresh h := (last (null .+ 1) (get_highs h)) .+ 2.

Lemma lowPtUn A1 A2 h1 h2 (x : ptr) (v1 : A1) (v2 : A2) :
def (x :-> v1 :+ h1) ->
def (x :-> v2 :+ h2) ->
(x :-> v1 :+ h1 =~ x :-> v2 :+ h2) = (h1 =~ h2).