CREATE TABLE test(
  id int,
  parent_id int REFERENCES test(id)
);

is equivalent to

CREATE TABLE test(
  id    int,
  parent_id int,
  FOREIGN KEY(parent_id) REFERENCES test (id)
);

However, only the latter is fully read by our grammar parser. If you use the first version, DB4S will work but silently drop the foreign key clause when you edit the table definition.