CREATE TABLE "br_railbuses" (
  "number_range" text,
  "builder" text,
  "introduced" real,
  "no_built" real,
  "region" text,
  "withdrawn" real
);

Question: when was the building withdrawn when the builder was ac cars and was introduced before 1958?
SQL: SELECT SUM("withdrawn") FROM "br_railbuses" WHERE "builder"='ac cars' AND "introduced"<1958;