(** This function is a generalisation of [hlist_gen] in which the function [f]
    takes the additional parameter [member a ls]. **)
  Fixpoint hlist_gen_member ls : (forall a, member a ls -> F a) -> hlist F ls :=
    match ls as ls return ((forall a : A, member a ls -> F a) -> hlist F ls) with
    | nil => fun _ => Hnil
    | a :: ls' => fun fm =>
        Hcons (fm a (MZ a ls'))
          (hlist_gen_member (fun a' (M : member a' ls') => fm a' (MN a M)))
    end.

Lemma hlist_gen_member_hlist_gen : forall ls, hlist_gen_member (fun a _ => f a) = hlist_gen ls.