(* the three projections from defPtUn are often useful *)

Lemma cancel A h1 h2 x (v1 v2 : A) :
def (x :-> v1 :+ h1) ->
x :-> v1 :+ h1 = x :-> v2 :+ h2 -> [/\ v1 = v2, def h1 & h1 = h2].