I have always thought myself to be a multi-faceted programmer, finally there is proof.
The issue of combining facets has been on the table for quite some time now. I have finally had some time to take a shot at it. The basic machinery underneath is now in place, I am not as convinced about the UI. This version has been installed on the Aquifer Portal Test System, soon to the production server , try it and let me know what you think.
Now when you breakdown a search result screen then select one of the displayed facets, it becomes a filter at the top of the facets sidebar. Further faceting and facet selection can add additional such filters and the filters can be removed individually. This type of interface was borrowed at least partially from the BlackLight project. As suggested by Steve Toub, when a decade is selected the faceting window is changed to be year, suggesting at least some hierarchy there. When removing a filter the faceting window is returned to the facet set from which the filter came. Empirically that seemed the most logical behavior.
It might be possible to add the feature via a button or something that would allow selecting the NOT of a facet, i.e. filter the search to only records that do not have a particular facet value. I could try this if it seems like it would be a useful searching tool, but it might just be screen clutter.
One confusing aspect of faceting is that even when a particular facet is selected the facet list continues to show other facets. This is because the records retrieved with the given filter most likely have other values in the facet list in addition to the filter selected. Maybe this is obvious, it's pretty obvious to me but this is the kind of issue that I always want to check against the principle of least astonishment, if I get confused users may be more so, but perhaps not.