CREATE TABLE table_name_49 (laps INTEGER, driver VARCHAR(255), grid VARCHAR(255), constructor VARCHAR(255))
What is the average Laps for a grid smaller than 17, and a Constructor of williams - bmw, driven by jenson button?
    
SQL: SELECT AVG(laps) FROM table_name_49 WHERE grid < 17 AND constructor = "williams - bmw" AND driver = "jenson button"