In the papers classified as ML, how many cited butz01algorithmic?
classification refers to class_label; paper cited by refers to citing_paper_id; citing_paper_id = 'butz01algorithmic';
SELECT COUNT(T1.paper_id) FROM paper AS T1 INNER JOIN cites AS T2 ON T1.paper_id = T2.citing_paper_id WHERE T1.class_label = 'ML' AND T2.cited_paper_id = 'butz01algorithmic'