@ARTICLE{HaehnleSchmitt94,
        AUTHOR             = {Reiner H\"{a}hnle and Peter~H. Schmitt},
        JOURNAL            = {Journal of Automated Reasoning},
        PAGES              = {211--221},
        TITLE              = {The liberalized $\delta$--rule in free 
                              variable semantic tableaux},
        VOLUME             = {13},
        YEAR               = {1994}
}