question: How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?
answer: SELECT SUM(total_produced) FROM table_name_13 WHERE wheel_arrangement = "b-b" AND prr_class = "bs6" AND builder’s_model = "ds-4-4-660"
context: CREATE TABLE table_name_13 (total_produced INTEGER, builder’s_model VARCHAR, wheel_arrangement VARCHAR, prr_class VARCHAR)