Composite index seek

A composite B-tree index is keyed on (region, score) and stores entries sorted by region ascending, then score ascending. Entries are given as [region, score, row_id], already in that sorted order. Implement an index seek for the query WHERE region = R AND score >= min_score, returning the row_ids of matching entries in index order. You must use the index ordering to seek — binary-search to the first matching entry rather than scanning all entries (assume the matching slice is small relative to the index). region is a string, score an int. Up to 200000 entries.