ACTION-324: Ping carlos/eric in 2-3 weeks for progress on Fed query issues

- Examples in the Fed document are very domain specific, to generalise them.
- Failed execution proposals:
I) make it explcicit if you want to allow failure (i.e. SERVICE SILENT ,or alike?)
I am personally not a big fan of having a separate behavior for OPTIONAL{ SERVICE ... }
(which will be hard to define if SERVICE is deeply nested within an OPTIONAL...

II) disallowing failure alltogether, which I'd find very limiting... (with a special error code in the
protocol indicating "failed SERVICE call"?

III) a FILTER operation that allows to check whether a service is "active"?
isActiveService(?s) ? otherwise, how can I execute a SERVICE subquery only for
those services which correspond in fact to SPARQL EPs... at this point though, I'd be unclear of
how to define the semantics with respect to that FILTER

- Rewrite document in a formal style
- Define semantics
- Clarify Greg's comments about Certainly/potentially bounded
-