add support for lists to expr_build_next

template <typename... T>

ExprStorage expr_build_next(ExprStorage head, T&&... tail) {

  auto n = expr_build_next(std::forward<T>(tail)...);

  if (!head) {

    return n;

  auto e = head.get();

  while (e && expr_next(e)) {

    e = expr_next(e);

  }

  auto e = std::move(head);

  expr_set_next(e.get(), std::move(n));

  return e;

  if (e) {

    expr_set_next(e, std::move(n));

    return head;

  } else {

    return n;

  }

}

template <typename... T>