Is it possible to set a property for the whole index?

[Qizhen Ruan]

Hi,

I need to set a property, "language", to all documents in the index.  But
for a single index, the language for all documents are the same.
Is that possible that I create a property for the index file instead of for
each document in it?
When I got the result from from multiple indexes search, could I know which
index the result belongs to?

Thanks,
Qizhen