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

Lemma updi_iinv x xs1 xs2 h1 h2 :
size xs1 = size xs2 -> def (updi x xs1 :+ h1) ->
updi x xs1 :+ h1 = updi x xs2 :+ h2 -> xs1 = xs2 /\ h1 = h2.