We introduce a general model of random context multiset grammars as well
as the concept of multiset random context checkers and transducers. Our main results
show how recursively enumerable sets of finite multisets can be generated using these
models of computing; corresponding results for antiport P systems are established, too.
It is a well-known fact that the time of execution of a (biochemical) reaction
depends on many factors, and, in particular, on the current situation of the whole system.
With this motivation in mind, we propose a model of computation based on membrane
systems where the various rewriting rules have different times of execution and, moreover,
the time of execution of each rule can vary during the computation, depending on the
configuration of the whole system (in this sense, the computation is "time-driven"). We
show that such systems are universal in a very simple framework: a regular time-mapping
suffices to obtain universality for systems with minimal cooperation (one catalyst).
