Natural language constructions can require even more complex forms such as anbnc n, which in turn must be defined using indexed grammars and parsed with a machine that has the equivalent of multiple stacks of memory.
"suppose that we had access to hardware that would handle FSTNs [finite-state transition networks] ... ultrafast and observed that in actual occurrences of anb n constructions, the value of n never exceeded 3; then we might decide to compile our RTN [recursive transition network] ... descriptions down into FSTNs subject to an n = 3 upper bound (such a compilation is possible for any given finite upper bound on the value of n)" (Gazdar and Mellish 1989).