In Chapter 6 we address the general questions of the last section, and then make some suggestions for future research; in particular, we mention some partial results that could not be easily fit into the main chapter development, namely the application of circuit methods to time-varying systems in §6.2.7 (and an application to time-varying vocal tract modeling is indicated), and a possible foundation for a theory of the boundary termination of passive distributed networks in §6.2.3.