search for: set_sort_by_

Using MultiValueSorter, I can sort by key1, key2, relevance; or relevance, key1, key2. But AFAIK, I can't sort by key1, relevance, key2. Unless I spool out the entire result set or write some C++. I wonder if we need a new 'sort by' function that accepts any combination of keys and relevance in any order? The function would make it's own optimisations (ie is relevance first or