Name the coins that were not opened on May 2013.
not opened refers to open IS NULL; on May 2013 refers to year(date) = 2019 AND month(date) = 5
SELECT T1.name FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE STRFTIME('%Y-%m', T2.date) = '2013-05' AND T2.open IS NULL