Empty compound steps

Currently, we allow empty compound steps. Is that really necessary? I
think it'd be clearer if we required that a compound step contain at
least one step in its subpipeline.

The only use case that I recall is generated content. If that's really
a problem, I think I'd rather have an atomic p:nop component that does
nothing.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | All our foes are mortal.--Paul Valéry
http://nwalsh.com/            |