  Recently, we have proposed two complementary approaches, top-down and
bottom-up, to multilevel supervisory control of discrete-event systems. In this
paper, we compare and combine these approaches. The combined approach has
strong features of both approaches, namely, a lower complexity of the top-down
approach with the generality of the bottom-up approach. We show that, for
prefix-closed languages, a posteriori supervisors computed in the bottom-up
manner do not alter maximal permissiveness within the three-level coordination
control architecture, that is, the supremal three-level
conditionally-controllable and conditionally-normal language can always be
computed in a distributed way using multilevel coordination. Moreover, a
general polynomial-time procedure for non-prefix closed case is proposed based
on coordinators for nonblockingness and a posteriori supervisors.
