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 W on the set W, where W is the union of the sets W, 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".