CREATE TABLE table_204_639 (id DECIMAL, "poll company" TEXT, "source" TEXT, "publication date" TEXT, "psuv" DECIMAL, "opposition" DECIMAL, "undecided" DECIMAL)

which source was used before may 2010 ?
SELECT "source" FROM table_204_639 WHERE "publication date" < (SELECT "publication date" FROM table_204_639 WHERE "publication date" = 'may 2010')
SELECT "source" FROM "table_204_639" WHERE "publication date" < (SELECT "publication date" FROM "table_204_639" WHERE "publication date" = 'may 2010')