The roundup "filter" method (see hyperdb.py) has a "search_matches"
parameter that can be used to constrain the search.  In particular,
only elements which otherwise match the search *and* are in
search_matches will be considered.

Conceptually, 'search_matches' is a set. However, as some backend code
calls 'has_key()' on it, it needs to be a dictionary.

The attached patch simplifies this, so any sequence type may be used.

OK to check in ?