ex provides a <b>primary filter</b>
 for range rectangle queries.  The various query methods return a list of
 all items which <i>may</i> intersect the query rectangle.  Note that
 it may thus return items which do <b>not</b> in fact intersect the query rectangle.
 A secondary filter is required to test for actual intersection 
 between the query rectangle and the envelope of each candidate item. 
 The secondary filter may be performed explicitly, 
 or it may be provided implicitly by subsequent operations 