Current transaction builders make adding links tricky

[librarianmage]

Each TransactionBuilder currently initializes its transactions with data.EMPTY_SET, which is a frozenset. As such, adding links can’t be done in place, but the current framework only provides in-place manipulation hooks.

[librarianmage]

perhaps the parameter could be self.get_links() or data.EMPTY_SET or set(self.get_links()) or data.EMPTY_SET or something