Consider a site with a set of “checkbox navigation” groups on a search result page. To let the user drilldown the search result.
Say we were searching for the phrase Programming.
Checkbox navigation would the be, for example
Blog (12) 
MVC (4) 
Messaging (3) 
April 2014(2) 
May 2014(9) 
Jun 2014 (14) 
The whole idea is, as I said, to be able to drill down the search result to say articles matching “Programming”, tagged in category MVC – AND written in April 2014 or May 2014.
The checkboxes inside each group are OR:ed together and AND:ed with other groups. Combined with original search phrase it limits the actual documents returned.
However – when the user clicks on say MVC – then of couse the DATE options – and numbers – update so it matches the correct number. I.e the number is to be though of as a expectation. If i click this checkbox as well I will be given X more documents (hrmm, not entirely true as one document could have multiple categories etc). But my main point is that calculation of facets in each respective group should not include itself in the filter .
This is (better) described at the SOLR site: http://wiki.apache.org/solr/SimpleFacetParameters – scroll down to Tagging and excluding Filters
ANyway, since I have yet to fiond an extension library to Lucene.net offering this kind of facet behaviour I decided to implement it myself.
So here it goes: MultiFacetLueceneNet on github