What is the sum of low estimate(s) that is listed for the date of 1978?
CREATE TABLE "human_sacrifice_and_mass_suicide" (
  "low_estimate" real,
  "high_estimate" real,
  "type" text,
  "location" text,
  "date" text
);
SELECT SUM("low_estimate") FROM "human_sacrifice_and_mass_suicide" WHERE "date"='1978';