1. An indexing method comprising: deriving from a corpus of segments a hierarchical index that indexes events of a selected event type occurring in segments of the corpus at a frequency higher than or equal to a threshold frequency, the hierarchy relating events by an order structure in which coarser events subsume finer events, the hierarchical index further indexing at least some low frequency events that occur in segments of the corpus at a frequency lower than the threshold frequency; and processing a query respective to a queried event, the processing referencing information stored in the index relating to either (i) the queried event if the queried event is indexed or (ii) a coarser event that is indexed and that subsumes the queried event if the queried event is not indexed.