Remark of_list_unchanged:
  forall k l m, ~In k (map fst l) -> T.get k (List.fold_left f l m) = T.get k m.

Lemma of_list_unique: forall k v l1 l2, ~In k (map fst l2) -> T.get k (of_list (l1 ++ (k, v) :: l2)) = Some v.