@TECHREPORT{nonnengartMPI96aux,
        AUTHOR             = {Andreas Nonnengart},
        ADDRESS            = {Saarbr{\"u}cken, Germany},
        INSTITUTION        = {Max-Planck-Institute for Computer Science},
        MONTH              = {June},
        NUMBER             = {MPI-I-96-2-005},
        TITLE              = {Auxiliary Modal Operators and the Characterization of Modal Frames},
        YEAR               = {1996}
}