Root factory gets defined: "A root factory returns a model instance." This was
very helpful.

p. 162: "Specifying an interface instead of a class as the context or
containment arguments within view configuration statements effectively makes it
possible to use a single view callable for more than one class of object." Why
is it desirable to use a single view for two different types of models? Why not
just write two separate views?

p. 163: "A model class may implement zero or more interfaces." Under what
circumstances would it be necessary for a model class to implement more than one
interface?