What are the first names of all history professors who do not teach?
select t1.emp_fname from employee as t1 join professor as t2 on t1.emp_num = t2.emp_num join department as t3 on t2.dept_code = t3.dept_code where t3.dept_name = 'History' except select t4.emp_fname from employee as t4 join class as t5 on t4.emp_num = t5.prof_num