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The concat is to make sure the shelf browser references the correct items.
cn_sort is not unique, but should be unique for a title.  Then the itemnumbers
are unique within that title.  By doing the concats, we can pick the unique
items
on either side of the physical shelf, and be sure that the specific copy of the
book
that is showing a link to the shelf browser will be the one in the middle of
the displayed
set. 

So 1/ index location column
2/ consider implementing shelfbrowser as a separate table for the moment,
since we shouldn't really be adding columns to the items table at this point.