for continuous-time systems (see below) completeness will always mean Q-completeness, where Q is the class of all essentially bounded measurable controls.
the nontriviality and restriction axioms in the definition of system are automatically true if the system is complete.
in that case, a system is precisely the same as an action of Z on the set Z, where Z is the union of the sets Z, thought of as a semigroup with a partially defined binary operation (concatenation).
many of the concepts that we study are direct analogues of those studied for semigroup or group actions; for instance, "controllability" will be the analogue of "transitivity".