Can you see the name of the document being loaded?

[Jonathan Coveney]

I want to calculate some statistics on a per document basis, and it seems
like the only way to do this would be to emit a compound key of
(key,documentname).
1) Is this the case, or is there a better way to do this?
2) If this is the only way to calculate a per input file basis, where is the
right place to grab this? A custom line reader? What object is exposed to
this?