The interface allows factoring out getting the next sequence from
a generator. A generator has a lot of boilerplate code to create
the row and its beans, and has internal state information about the
sequence it is generating. So the abstraction allows a generic
generator to be wired to a sequence bean which implements this
interface. This isn't perfect, since it assumes there's only one
value to return per row. Eventually I will need to create a more
sophisticated and generic interface.