Use the predecessor tree to find dependencies based on outputs.

        Returns: A list of transaction ids.

def _find_output_dependencies(self, outputs):
        """Use the predecessor tree to find dependencies based on outputs.

        Returns: A list of transaction ids.
        """
        dependencies = []
        for address in outputs:
            dependencies.extend(
                self._predecessor_tree.find_write_predecessors(address))
        return dependencies