Combine facets into one facet

In some cases it makes sense to enhance or filter a facet's result items. For example to exclude some items. At the moment we already have a filter to "ignore" hard coded lists. But it would be great to have a dynamic solution here.

I suggest to add a processor to "combine" multiple facets (union, diff, intersect). The facets could base on different sources. This way I could configure two different searches using different sophisticated filters or different datasources (one visible search result, one hidden) and combine the results:

• Hierarchical Taxonomy but only terms that have been added by a specific user or within the last year.
• Entity creation dates but only Mondays

Using Solr you can do such things natively using Facet Queries. But we don't have a frontend in Drupal for that. The Facet module only knows about "fields". So this is an approach to offer something similar to the Search API ecosystem without beeing limited to a specific back-end.