user: Question: Which leader has been a leader since 2007?
Columns: ['Leader', 'Party', "Leader's Seat", 'Number of Seats', 'Leader since']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Peter Robinson', 'Democratic Unionist Party', 'Belfast East', 38.0, '31 May 2008']
SQL:

assistant: SELECT "Leader" FROM "Table" WHERE "Leader since" = '2007';