Heterogeneous cosimulation: Ptolemy
Any model can be used at the top of hierarchy. Within each level of hierarchy, it is possible to have Blocks containing foreign domain.
Hierarchy heterogeneity is quite different from the concept of a simulation backplane (that imposes a top-level models of computation through which all subsystem interact).
Active objects in a Domain are Stars. They perform computation and communications with other objects through PortHoles.