find the football match, with a start date less than or equal to 2010-06-14t20:30:00-08:00, where héctor vergara was an assistant.
(AND soccer.football_match (AND (JOIN (R soccer.football_referee.assistant_referee_at) m.02z274l) (le time.event.start_date 2010-06-14T20:30:00-08:00^^http://www.w3.org/2001/XMLSchema#dateTime)))