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

Lemma highPtUn2 A1 A2 h1 h2 (x1 x2 : ptr) (v1 : A1) (v2 : A2) :
def (x1 :-> v1 :+ h1) ->
def (x2 :-> v2 :+ h2) ->
high x1 -> high x2 ->
h1 =~ h2 -> x1 :-> v1 :+ h1 =~ x2 :-> v2 :+ h2.