user: Question: Which constructor had a Time/Retired of Collision, where the grid number was less than 18 and Nick Heidfeld was the driver?
Columns: ['Driver', 'Constructor', 'Laps', 'Time/Retired', 'Grid']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['Ralf Schumacher', 'Williams - BMW', 69.0, '1:34:31.522', 2.0]
SQL:

assistant: SELECT "Constructor" FROM "Table" WHERE "Time/Retired" = 'Collision' AND "Grid" < 18.0 AND "Driver" = 'Nick Heidfeld';