Of course, one unit tests above was also broken. The fixed unit test suite:

    (suite join-tables
            empty (assert-same (join-tables)
                               (obj))
            one-table (assert-same (join-tables (obj 1 2))
                                   (obj 1 2))
            two-tables (assert-same (join-tables (obj 1 2) (obj 2 3))
                                    (obj 1 2 2 3))
            multiple-values (assert-same (join-tables (obj 1 2 2 3)
                                                      (obj 3 4 4 5))
                                         (obj 1 2 2 3 3 4 4 5))
            three-tables (assert-same (join-tables (obj 1 2)
                                                   (obj 2 3)
                                                   (obj 3 4))
                                      (obj 1 2 2 3 3 4))
            overrides-values (assert-same (join-tables (obj 1 2)
                                                       (obj 2 3)
                                                       (obj 3 4)
                                                       (obj 1 0))
                                          (obj 1 2 2 3 3 4)))