Results in the "xproc-step" namespace

Hi all,

We have a number of custom XProc steps, some of which use c:result in
the output document. Now I am thinking that the
"http://www.w3.org/ns/xproc-step" namespace is maybe meant only to be
used with the standard steps, and not with custom steps. I am wondering
whether what we are doing right now is actually correct...

So, say you are implementing a custom step that needs to return a URI
string (such as in the case of p:store), what is the best thing to do:

1. Use c:result as much as possible to promote consistency
2. Don't use c:result at all (because it can be used only with standard
steps) and invent your own vocabulary
3. It doesn't matter

If any of the options is preferred over the others, do we need to say
anything about it in the spec?

Regards,
Vojtech