Implement missing function

        return [item for group in self.groups for item in group.items]

    def item(self, id: str) -> BudgetaryItem:

        pass  # TODO

        matching_items = [item for item in self.items if item.id == id]

        match len(matching_items):

            case 0:

                raise KeyError(f"No budgetary item with id = {id}")

            case 1:

                return matching_items[0]

            case _:

                raise RuntimeError(

                    f"There were more than 1 budgetary item with the same id, this should not happen"

                )

    @property

    def as_dict(self) -> dict[str, Any]: