Find the first names of professors who are teaching more than one class.
SELECT T2.emp_fname FROM CLASS AS T1 JOIN employee AS T2 ON T1.prof_num  =  T2.emp_num GROUP BY T1.prof_num HAVING count(*)  >  1
first names of professors = emp_fname; teaching more than one class refers to HAVING COUNT(*) > 1; professors are identified by emp_num which is equivalent to prof_num in the class table;