How do I implement facets using Zebra's SRU interface?

I have implemented a Koha catalog, the underlying indexer is Zebra, and I have turned on the SRU interface. See, for example, the simplest of SRU Explain responses against the catalog:

http://catalog.distantreader.org:2100/biblios
  
I can then do a search for love, request 8 records, and ask the result to be formatted as MARCXML:

http://catalog.distantreader.org:2100/biblios?version=2.0&operation=searchRetrieve&maximumRecords=8&recordSchema=marcxml&query=love
  
'Works great, in fact, it is VERY fast!

With version 2.0 of the SRU standard [1], returning faceted results seems to be a choice, but when I add faceting parameters I see no facets in my results. For example:

http://catalog.distantreader.org:2100/biblios?version=2.0&operation=searchRetrieve&maximumRecords=8&recordSchema=marcxml&query=love&facetLimit=8

To what degree is my request invalid and/or to what degree does Zebra not support facets? For example, I get no facets with I query the LOC:

http://lx2.loc.gov:210/LCDB?version=2.0&operation=searchRetrieve&maximumRecords=8&recordSchema=marcxml&query=love&facetLimit=100&facetStart=2&facetSort=recordCount

But then maybe the LOC is using Zebra?


[1] SRU standard about facets - http://docs.oasis-open.org/search-ws/searchRetrieve/v1.0/os/part3-sru2.0/searchRetrieve-v1.0-os-part3-sru2.0.html#_Toc324162452

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Eric Morgan